Nice job! I can appreciate all of your effort because I'm essentailly doing the same thing to a 50 Desoto convertible. Same goal of building a nice cruiser, not a race car. Mopar 383, 727 auto, 2000 Explorer disc rearend. Four wheel disc brakes. Also keeping the original steering box and linkage.
The Desoto is very similar to your car so your engine compartment and so are the challenges of fitting a Mopar with much the same solutions as yours.
To maximize passenger room I was more conservative with the firewall and only cut pockets for the heads. I dropped the engine a little lower with a truck pan and remote oil filter.
One thing I did to get space for the starter was to fabricate a 1/2" steel spacer that I inserted between the the steering box and its frame mount. That moved the pitman arm over the the left, away from the starter by a 1/2".
I also offset my engine to the right around 2". One limitation with a convertible is that it's an X frame with a closed steel tunnel in the center that the drive shaft goes through. That was a problem with the Explorer rearend due to the rear's pinion being offset about 3" to the right. Driveshaft hit the side of the frame tunnel. To solve this, I centered the rear's pinion, actually it's now offset about 1/2" to the left. To center I basically cut the axle tubes at a specific length and swapped them side to side. That allowed the axles to be swapped. Rearend is the same overall length but the pinion is now more centered.