A well built TH350 or Ford C4 could easily handle 300 HP and weigh less. Really don't see the need for a TH400, but If that's what you then go for it.
Do a search on block adapter plates, specifically those used to mate Buick-Olds-Pontiac engines to Chevy autos. There is a similar plate used to mate AMC V8's the Chevy autos too. A plate like that is what you're looking for. I suspect that you'll have to build it.
A gotcha that's bitten me twice, the GM engine to trans alignment dowel pins are 1.000 inch ABOVE crank centerline, not on it.
You'll have to use what ever stock Toyota flexplate is available for the engine. Have someone like TCS mod the Toyota converter with the GM snout & splines. Would advise talking to them b4 investing much time or resources in the project.