Depends if you own any parts and what size the budget is. There are a lot more choices for RB intake manifolds. Headers can be a problem with B engines depending on the heads that you use.

My "go to" combo is a 440 block with a 4.25 crank and 6.80 long Chevy rods. That gives you a very foolproof 505 with fairly inexpensive parts. If you're using a standard head like an Edelbrock RPM then you can easily make 650/650 if you're willing to pay for porting. You'll want to build a zero deck engine with a bit of a dish to get the compression just right.

A big flat tappet cam should get you there and saves money over a roller setup.