5.9L Magnum questions
A couple of things to consider:
The '66 transmission has the small support nose on the convertor, the Magnum has the large diameter crank pocket. You will need a spacer/adapter to run an early style convertor on the late motor. Many convertor suppliers may have issues supplying an early style convertor. No, a later '68 and up convertor will not fit an early 904- different input shaft splines. Consider finding a later ('68 and up) 904 to mate to your Magnum- no adapter bushing needed, virtually unlimited convertor choices, and all the hardware (shift linkage and levers, cooler lines, speedo drive) is a direct swap-over from your '66 904. If you use a '904 from the early/mid '70s you will have the added benefit of a valve body with part-throttle kickdown.
Linkage for the Magnum-
The early As used a kickdown linkage with a bellcrank that bolted to the rear of the driver's side head with 2 bolts; the Magnum only has one bolt in this location, so a means of stabilizing the bracket will be needed. A small added bracket or "finger" to locate it to a convenient section of the head is all that's needed.
The '66 (and earlier) 273 had a throttle cable clamp that bolted to a boss on the intake manifold, aftermarket Magnum intakes don't have this boss so you'll have to get creative with a later throttle bracket.
Throttle Bracket for Magnums
This method may also allow you to use a later kickdown linkage.
I'm not sure about your '98 Magnum, but somewhere around that time they switched to a one-piece balancer/pulley. If you have that, you'll need to switch it out for an earlier two-piece unit that will allow you to bolt on the early LA pulleys. You will need to oblong one hole in the early LA pulley for it to fit the balancer- a five minute project with a rat-tail file. You will also need to re-mark your balancer for the early LA passenger side timing marks.
Speaking of filing an oblong hole, you'll need to do that to one hole of the Magnum flexplate also to get the 904 convertor to fit. You need to run the Magnum flexplate due to it's unique balance.
As far as the exhaust manifolds, I assume you're referring to using the early A manifolds. Yes, they'll fit- sometimes with just a thick exhaust gasket on the driver's side, sometimes you'll need to go with the header flange. Just mock it up ahead of time and check your clearances. If you don't have power steering, you can get Magnum manifolds to work. Passenger side drops right in, and the drivers side will need a bit of shimming/grinding but has been made to work.
And regardless of which way you go with exhaust (manifolds/headers) remember that you'll need to modify the transmission crossmember to clear a dual exhaust.
Dual Exhaust Crossmember Modification on 64-66 A-Bodies?
The motor mount brackets from the early A will fit the Magnum, but will need a bit of work to do so.
(Photos posted by @65 Cuda 340 )
Modified passenger side bracket
And while we're talking about motor mounts, you might want to consider this (especially since correct early A biscuits are becoming increasingly difficult to find):
https://earlycuda.org/techdocs/JimBowenTheHolyGrailofMotorMounts.pdf
That's all I got for now.
A couple of things to consider:
The '66 transmission has the small support nose on the convertor, the Magnum has the large diameter crank pocket. You will need a spacer/adapter to run an early style convertor on the late motor. Many convertor suppliers may have issues supplying an early style convertor. No, a later '68 and up convertor will not fit an early 904- different input shaft splines. Consider finding a later ('68 and up) 904 to mate to your Magnum- no adapter bushing needed, virtually unlimited convertor choices, and all the hardware (shift linkage and levers, cooler lines, speedo drive) is a direct swap-over from your '66 904. If you use a '904 from the early/mid '70s you will have the added benefit of a valve body with part-throttle kickdown.
Linkage for the Magnum-
The early As used a kickdown linkage with a bellcrank that bolted to the rear of the driver's side head with 2 bolts; the Magnum only has one bolt in this location, so a means of stabilizing the bracket will be needed. A small added bracket or "finger" to locate it to a convenient section of the head is all that's needed.
The '66 (and earlier) 273 had a throttle cable clamp that bolted to a boss on the intake manifold, aftermarket Magnum intakes don't have this boss so you'll have to get creative with a later throttle bracket.
Throttle Bracket for Magnums
This method may also allow you to use a later kickdown linkage.
I'm not sure about your '98 Magnum, but somewhere around that time they switched to a one-piece balancer/pulley. If you have that, you'll need to switch it out for an earlier two-piece unit that will allow you to bolt on the early LA pulleys. You will need to oblong one hole in the early LA pulley for it to fit the balancer- a five minute project with a rat-tail file. You will also need to re-mark your balancer for the early LA passenger side timing marks.
Speaking of filing an oblong hole, you'll need to do that to one hole of the Magnum flexplate also to get the 904 convertor to fit. You need to run the Magnum flexplate due to it's unique balance.
As far as the exhaust manifolds, I assume you're referring to using the early A manifolds. Yes, they'll fit- sometimes with just a thick exhaust gasket on the driver's side, sometimes you'll need to go with the header flange. Just mock it up ahead of time and check your clearances. If you don't have power steering, you can get Magnum manifolds to work. Passenger side drops right in, and the drivers side will need a bit of shimming/grinding but has been made to work.
And regardless of which way you go with exhaust (manifolds/headers) remember that you'll need to modify the transmission crossmember to clear a dual exhaust.
Dual Exhaust Crossmember Modification on 64-66 A-Bodies?
The motor mount brackets from the early A will fit the Magnum, but will need a bit of work to do so.
(Photos posted by @65 Cuda 340 )
Modified passenger side bracket
And while we're talking about motor mounts, you might want to consider this (especially since correct early A biscuits are becoming increasingly difficult to find):
https://earlycuda.org/techdocs/JimBowenTheHolyGrailofMotorMounts.pdf
That's all I got for now.