First off..I speak with experience.
You don't want an adapter plate. They are lots of trouble. Starter engagement, getting everything indexed properly is a chore, and the amount of time and money spent making it work is usually not worth the trouble. Not to mention a stock powerglide bellhousing is not the strongest bellhousing in the world. Notorious for cracks.
I would highly suggest a bellhousing style adapter, the kind where the OEM powerglide housing is removed totally. This way you usually don't have alignment issues. Also don't have starter problems and the bellhousing adapter is typically SFI approved which kills 2 birds with one stone. Some people don't like them, however, due to the fact that the pump bolts hold the pump in AND hold the bell to the pump/case. Big deal. There's 7 bolts all are 5/16"...it's plenty strong and rated for something like 2000 HP from what I'm told. Beyond those power levels I'd be looking at a full SFI case with the SBF bellhousing...they're available with all poplular bellhousings from Mopar, VW, SBF, Ford Modulars, etc. I used an Ultrabell after screwing around with adapter plates. Totally easy and it removed the stock cracked PG bellhousing.
Finally, those who say a PG isn't worth having, most of ya haven't run one in a dedicated drag car. For a street car, a PG SUCKS-no doubts about it. You want 3 or more speeds for anything that's goign to be run on the street. But for drag racing, the PG is very hard to beat. Tough. Dependable. Cheap to build/rebuild, easy to build/rebuild at home. Easy to diagnose problems if there is any. Repeatable. Good for turbo cars. Good for bracket cars. Good for all out heads up cars that must run an auto trans. Good for cars under 3000 lbs. Good for cars over 3000 lbs too if the converter it set up properly. Once again...speaking from experiences. Ran C4's for many years. Tried a 'Glide in 1998 and never went back to the trusty 3 speed. Have run the PG's in footbrake, transbrake, low HP, lower HP, big tires, small tires, light car, heavy car, 2 gears, high gear only and lots in between. It's always been a better transmission for drags. Tried one on a street car...and yes they suck on the street with no OD and that 1.82/1.76 first gear (close to the same SECOND gear ratio in a 3 speed auto).
There is a reason you see more PG's at the drags than any other type trans....