Hi Rev,
I have an a518od in a 71 Duster. Its mostly a street car and I love the od. The install required alot of floor mods tho. I got 16+ mpg running a steady 80/85 on the interstate on a AZ to MN trip last summer. However for strictly performance application It might not be the best choice. Changing out the 727 I slowed from 14.68 to 15.00 (360 with a 3.23 rear). Probably the added weight as well as hydraulic and rotational losses associated with the OD section.
I got a lot of info from SMR trans,
www.smrtrans.com. The A518 is for a smallblock only but conversions are available. On Moparts.com there is alot of photos of a conversion using a JW Ultrabell. Look in tech for "BIG BLOCK A518 OVERDRIVE CONVERSION".
Transtech Transmission Specialists of Dubuque IA will weld on a BB bellhousing from a donor 727.
Back in the June 07 Mopar Action mag they put Keislers set up in a 69 Road Runner. They comment on the lower 1st gear helping out too. The only floor pan mod they mention is to roll over the pinchweld at the floor to cowl point and a few hammerblows to the right side tunnel.
I did all the work myself so the 518 was the cheapest route for me, but If I had to pay I think the Keisler is good choice. A couple people I know have the GV od and they say they have a little more drivetrain vibration than before, but I bet the GV can handel more HP.