A518 vs 727

I'm pretty sure the use of the 200r4 in these cars is purely because they happen to fit better than the larger dodge trannies.
My car is set up for a 4l80e, plans are for a 572 HEMI. I want something with a lock up convertor and that is electronically controlled. I know there is a place in Canada that sells an adapter to put a 518 behind a big block but when I discussed this with my friend that will be building the trans for me some day he just laughed......I will not repeat the rest of it because I know how sensitive some folks are about certain things around here.....