The answer is split 50/50.

It all depends.

The GearVendors has been used for several years in motorhomes and it may well be possible to buy an existing setup. I've seen the overdrive units at swap meets in the $1000 range, the company will sell you the rest of the parts to make it work. If you're savvy enough to get ths far, getting the driveshaft modified is a piece of cake. Many longer wheelbase vehicles have two-piece driveshafts and those are very easy to shorten.

The Mopar overdrive automatic transmission is also an atractive option, cut off the bellhousing and swap on an aftermarket one. There are issues with the sttarter, but can be solved, thousands have.

Building the 518 to handle 440 power is no problem either, as the front if it is a 727.
BUT, the later auto trannys have been strengthened for use in front of Cummins and V10 engines, and I'd think of looking at those. Electronic controls sound daunting but I believe are easier to get right in the long run. Having a lockup converter would be nice, as well.

In conclusion, both ideas are attractive and both if done properly will cost about the same. I'd start looking at swap meets or CL and start talking to your favorite modern Mopar auto tranny shops to see which way the dice will land for you.

Also, Feets, thanks for keeping your eye on the ball about cost. That's important. I hope your transmission lasts a long time.

R.