j-d -- ! agree with most of your post.
Big block (383, 413, and 440) requires a different transmission bell housing than small block (318, 340, 360) engines.
While the internals are mostly the same, the aluminum case for big block and small block A727 automatic transmissions are different and not adaptable. Furthermore, big block A727s are becoming very difficult to find. (If you find a big block A727, save the case ... it can be rebuilt.)
Do not consider converting to a stock A518 automatic transmission. Early versions had issues with the lock-up clutches in the torque converter. (Same for A727 lock-up torque converters around the same time.)
There were (and probably still are) aftermarket torque converters. Likewise, Mother Mopar has re-engineered more recent versions.
The A518 is based on the A727, with the fourth (overdrive) gear in a tail cone added to the A727. The 'front' part of an A518 is basically an A727, with mostly identical internals. The A518 tail cone (and overdrive gear) was not as well engineered as the original A727. A better option is to add a Gear Vendors over/under drive unit to the 'back' of an A727.
