Well its easy to make 400 hp with vortec or other after market heads. If my memory serves me right that is more than enough to shake a corvair to peices. Unless you got major frame and drivline upgrade i would go as stock as possible with a v8. 290 would be plenty if not over kill. The original engine easily could be less than 90 sae hp. 330 sae hp could be as much as 4 times the cars designed output. Too much for stock frame or even boxed up stock stuff.
If you want efi i would consider a 80's vette motor nobody wants them and they make good tq and about 290 hp. Also great mpg with the th700r4. I would buy a corvette donor car swap everything over and sell whats left. Get a roller motor with tpi 88 or 89.
With all the issues with crate motors these days its hard to recommend. I would prefer to rebuild my own engines. A junk yard block that runs and moves can be the best option when the war chest is empty. A great car will wait for a great engine a car that does not move is not a great car.
Now not all crates got the same engine inside. If you are going to spend a few bucks to buy a jegs built motor might as well kick out the rest and buy an all new engine. Built with 100 percent new parts, block and heads. These engines for the most part are considered to be oem reliable.