slant 6 / 318 swap

The 63 dart has a 904 cable shift trans with a ball and trunion type u-joint.
You will be changing drive shafts to a slip yoke type with conventional u-joints. You will also be installing a floor shifter.

Schumacher has motor mounts that will bolt in a 318 or 360 onto the slant 6 k-member.

You will need a v-8 center link so the oil pan will clear.

The 727 trans is a larger trans and will not fit in the 63-66 A body without making a larger trans tunnel .

The late* 904 and 998 trans will bolt right in with a late 318 or 360**.
Note : *70-87
** Externally balanced requires a balance weight on the torque
converter .

As for 904 vs 727 in an early A body I suggest a built 904 with a Gear Vendors O/D.

'Which is stronger a 727 or 904 ?' A 727 is but it takes more power to operate it. Unless your planning on neutral starts at full throttle the 904 will get'er done.

With your combo a 8 1/4 rear axle will work fine and are available.

First thing to do is upgrade to Disc brakes. There are kits that use the early spindles or you could use the '73-76 A body spindles , rotors, calipers and upper control arms with ball joints . OR '73-78 B body Spindles, rotors,
calipers with the '73-76 A body upper control arms and ball joints.

This just in ... Unisteer is close to releasing a bolt in power rack and pinion
steering system for the A body.
This will open up header clearance on the drivers side.

As for headers there are a few choices TTi, Tri Y and Dougs. all are spendy.

But with the rack & pinion from Unisteer there may be another option for a simpler header similar to the original v-8 manifold except in tubing. Same goes for the passenger side.

Gee I wonder why no one has come up with a tube style header like the O.E. manifolds for early A body V-8 swaps. Oh sure it wouldn't flow like a tuned header but it would flow way better then the cast manifold and leave more room in the engine bay.