there's plenty of info on how to build huge horsepower Olds Diesel block based engines over on RealOldsPower.com If you've got the money go for it.
I would not do a Caddy 500 build (I almost went down that road), complete good running 500 motors are cheap and enticing but they don't rev, they just make a mountain of torque. Great for moving big old Caddys but not really a fun engine nor a race engine IMHO. There's some good recipies for BBOs out there; but it's gonna cost you a mountain of cash to build one up that will be reliable. Then there's the whole cost of beefing up everything else along with the BBO (Trans, rear end, etc.). If you have AC in the car, there might be slight clearance issue with the condenser that comes out of the firewall with the BBO too. Otherwise the BBO is a bolt in for the Gbody Cutlass.
I think the smart money is converting the DX using spacers and 330 crank. They aren't that uncommon I found one for my project within just a few weeks of looking. It's been ground, balanced, nitrided and polished and I think I've got around $600 into it total. That 330 crank won't give you the 442 cubes but it's a hell of a lot cheaper than the machine work involved using a 425 crank. It's the only Forged SBO crank that I know of. The N marked cranks are supposed to be decent as well. Mark Remmel has built serveral engines for people (Cutlassefi over on ROP) and sells parts (rods, pistons, cams etc.). I'm getting real close to wrapping up a 403 build based on one he did that was good for 450hp on a dyno. The SBO doesn't have the oiling issues the BBO does either, it's lighter which is nice in light car like the Gbody (well light compared to an Abody).