600 to 650 hp is fairly easy with a stock block and Trick Flow heads. Since it is a B body you might as well use a 440 block and build a 505. I'd go with TF 240 heads but you could step up to the larger port 270 heads if you want. If it was me I'd use a hyd roller cam and a throttle body type of fuel injection. The Sniper will support up to 700 hp and it is cheap but a Terminator kit gives you a lot more room for growth.

Transmission is a little tougher call. A good 5 speed manual setup makes it easier to have the right ratio for street driving as well as freeway cruising and going down the track but you have to put up with clutch maintenance. A 727 trans only has 3 gears so it is somewhat limited. You can adapt various OD autos or you can go with a bolt on overdrive. Neither of those solutions are very clean.

You'll have to be a lot more specific in terms of budget dollars and what your real goals are before you'll be able to make a decision.