I'd go w/ a 650 for a 340. Save the 750's for the big blocks or if you stroke it someday to 416. It's a little overkill for your combo (IMO). Bigger is not always (rarely) better in this case...
Don @ FBO recommended a 650 for my 340 and it's got 11.1 compression, ported heads 2.02/1/60's, .508 Purple cam and an Eddy RPM intake. They'll get one set up just right for your car.