I have read this thread several times and focused on understanding your goals.
I get it and I applaud your perseverance! Please press on. The folks who are interested will continue to support with information, questions and "atta boys". Those who are rubbed the wrong way will go silent.
The idea of an old car with updated and adequate power that fits, w/o major surgery, in the hole available sounds like a fun and satisfying project. It's something where you can learn while you solve the "build issues" and then enjoy the driving experience with something different.
All that said,.....I did the "shoehorn bogey" with no regrets. Not for you, however, one item that may apply to your search is that the stock radiator can be moved forward by several inches by simply reversing the "radiator hoop". It was the means by which Mopar accommodated the longer 6 cyl engine and that has been done by several on this forum. The extra inches may allow you to consider other engine options 'tho the Slant-6 is still too long.
Somehow I'd lean toward the Turbo 4 cylinder. Mercurys, Lincolns and I believe Fords had them. I'd guess you could get the engine, trans and computer in one package, maybe even get a whole car for cheap. The wrecking yards are full of them and I'd guess that parts and service are readily available. The challenge for me would be the electronics. 'Tho must be lotsa YouTube info on that. Success at clearing the electronics hurdle would be very satisfying.
Good luck. Don't take too bigga bite for your abilities. And, keep us posted.