Best option would be to use a 400 block (bigger bore). But, If you already have the 383 block, and aren't looking for a MAX effort, it should work just fine. You might cruise the local craigslist for a 400, though. I found a 400 long block out of a P/U not far from me for dirt cheap ($50 if I remember correctly!!).

Advantages to the B motor are plenty:
-lighter weight than an RB block.
-smaller physical size makes headers and high rise manifolds easier to navigate
-Same stroke rotating assembly in a "B" is substantially lighter in rotating mass than the RB combo i.e.: lighter pistons and lighter crank due to counterweights and main journals being cut down.
-While not true with a 383 block, a 400 blocks bigger bore helps with unshrouding the valves, lets it breath better.
-"B" blocks are generally understood to be a little stronger than "RB" blocks due to the lower piston sleeve.

The extra stroke will pay big dividends in trying to move a 4-door belvedere, which is a hefty sled to say the least. Bottom line, the 3.75 stroke build will stomp the same 3.38 build every time! and the bigger the car your moving, the bigger the advantage gets. My combo is a .040 over 400 block with a 413 RB crank ground on center, good for 452 cubes. It also has Stealth heads (with no port work) and a street friendly solid flat tappet cam and Pump gas allowable 10.3:1 CR. It moves my Satellite along pretty good (although on this board, you're NEVER fast enough) and in general I'm real happy with what I was able to build on my budget.

cutting the crank will be the only thing that is any different than a standard 383 build, and you'll want to find a machine shop that understands exactly what you're doing for this. Try to find a shop thats done it before. If you get the deer in the headlights look when you explain it, keep looking! You're looking for the guy that smiles and says "I've done that plenty of times!". This work shouldn't cost you much more than a few hundred bucks by itself.

Whatever you end up doing, Have fun!