63 dart v8 swap

Why do you need to modify anything on the firewall of a '63? The 63 and 64 are virtually identical under the hood and they put the 273 in the '64.
As you know you will need the dropped center link, either modified v-8 mounts from an early a-body or Schumaker conversion mounts.

And depending on which V-8.....

You need a center sump oil pan (not from a truck)
If you use the earlier iron water pump the stock radiator will work (if it has enough cooling capacity). Use a '65 water pump & your radiator lower hose lines up (360 water pump has lower hose on opposite side). If you use a 273 front cover & timing tab you will have to set TDC & mark the front dampener of a 360. The 360 timing cover marks are partially blocked when you use the early water pump.

Using the pulleys and alternator brackets from a 273 makes the job easy. (At least get them all from the same car so the pulleys line up) You will probably have to elongate one of the holes in the 273 crank pulley if you use a 360 as they have one hole offset.

You will probably want to use the 90 deg oil filter adapter from a 273.

Tap the air injection holes of later engines under the exhaust ports and plug with 1/8” pipe plugs.

Your biggest challenge will be exhaust. 273 manifolds don't flow the best. Headers are expensive. Not much else fits. Search for the discussions......

Modify the transmission crossmember to provide clearance for dual exhaust.

You will need a throttle pedal, cable, cable bracket and kickdown linkage(if using an auto) from a 273 car. You will probably require some individual modification to fit the new manifold.

Depending on what transmission you will use....
You can use a B&M flexplate (10239) to adapt a zero balance torque converter. If you want to use the early converter you will need a spacer as the pilot on the early converters is smaller. You can get that spacer ring from Torqueflite Patty. 206-365-1966. Another option is to put the front three main components from a 68 and later 904 into the early cable shifted 904, ending up with the later model input shaft, so you can use a later zero balanced torque converter. Or, have a custom torque converter made with the proper snout, input, and balance.

Clutch cars require a special flywheel with the counterbalance built in for a 360. (unless you zero balance the engine)

To use the larger bellhousing you will need a Z-bar from an A-body and shorten the cross shaft. Use the release lever from a B-body (11-7/8 OAL) with the fork pivot from the 273 bellhousing. Modify the z-bar bellhousing pivot to relocate the pivot ball.

I've probaly forgot several things. If I remember something else I'll add later.

What engine and tranny are you contemplating?