The A518 is basically an A727 with an overdrive unit in the tail housing. They can both be built to handle the same amount of torque by using basically the same parts. Either one can be built to handle 700 HP, but not for $800. The guys with pumped up Cummins powered trucks have pretty much solved any of the strength issues. An A518 will have some of the crap parts common to all later model Torqueflites like plastic servos. Sonnax makes nice billet replacements.
If you have 3.23 or taller (highway) gears, you wouldn't see much advantage. The nice thing about the overdrive is the ability to run 4.10 gears on the street and be able to cruise comfortably.
To put the A518 in the D150, it should be a pretty straightforward swap, requiring some rear mount mods and maybe shortening the driveshaft. To swap into an A-body requires moderate modifications to the unibody and rear mount and possible driveshaft shortening.
