I read here and there about guys using these Chevy trannys behind big block mopars. What's the advantage, and what's involved in the swap. I assume some kind of adapter plate is needed?

you need an adapter if you keep the GM case unless you cut the bell off and buy a Mopar bell.
A 130 tooth flywheel that bolts to you Mopar but allows use of the 400 converter. a mopar mini starter works in both cases. a rear trans mount modification and driveshaft. Most do it for the added strength over a 727 . that is always a point of dicussion around here. cheaper transmission to buy HP capability per dollar .better choice of converters

and if you run brackets a powerglide works great

i have a t400 and only did it for two reasons, Cheaper parts ( My converter was 400.00 cheaper because its for a GM not a T flite..) And the sprag can still break but is not as prone to roll over/break in a pedaling situation..

Its just a more widely used trans and in the long run a lot cheaper. Why i really cant say, just the way it is, Id rather use a 727 because its prob a lot lighter

I have one for sale on moparts in the race section if you are interested