I guess I am in the minority here. I agree with the few other folks who think you should do both. Keep the numbers matching motor, and buy or build a separate fun motor for now. My wife owns a C3 Vette, and though I'm not a huge Vette fan, I know many who are. The whole numbers matching thing seems foolish to me, but to people who think it matters, it REALLY matters. If you ever sell that car, assuming it's value rises in the next few years (and I'd bet it will, just sitting in the garage), the possession of the numbers matching engine might nearly double the value of the car, assuming other major components have matching numbers. Corvette restoration folks are really into this stuff. All that said, this was a period of awful performance for stock Vettes, and just about everything else as well. So, building up a separate motor, or getting a good crate motor, seems to me like a pretty good idea. The friends I have with street rods are all using GM crate motors of one kind or other. All small blocks, there are a couple of ZZ4s, a 330HP dressed-out and finished 350, as well as a 350 RamJet. Another friend is using a 454 Big Block Crate with an 8-71 blower. All of these owners are totally pleased with themselves for having made this simple, and warrentied decision. Instead of choosing one path, and possibly destroying the option for the second, I agree you should update the car, but keep the number-matching restoration option open for the future.
Pat