Tough question I must admit. It's so much up to personal preferences, and what is a "big" task and what is not. That varies with peoples experience, but nevertheless swapping the whole drivetrain is not a snap and will require a lot of work. Could be well worth it or then not. Anything is doable but it really is up to individual whether it makes any sense.
What caught my eye is that you consider modifying the floor for A518 something that you are not willing to do. That could easily be the easiest task along the whole project...
I recently put modern MPFI V6 with 5-speed standard and jeep rear in my '46. Lots of custom fabrication was needed, engine mounts, tranny mount, perches, clutch linkage, headers pipes etc. just to mention few. So my recommendation for you is to go with auto tranny, but not with modern RE42 that'll require computer.
Maybe 318 and 4-speed auto out of early 90's ram van? How about Aspen Volare Mirada from late 70's early 80's as a donor car? PM me your email and I'll shoot good instructions for installing LA into these puppys into your direction, someone here passed them couple years ago when I was planning my swap. Those are in word.doc format and cannot be posted here.
Whatever you decide, good luck, it will be fun with V8.