I would think the small block to big block conversion would be sort of a PITA. Aside from the mechanical (engine mounts, all accessories and brackets, hoses and connections, transmission, cooler lines, linkages, driveshaft, exhaust), if you want to stay EFI, you'll have some major mods to do in the fuel and electrical departments.
Depends on your goals I guess, but it's hard to believe that a well built small block can't meet your power needs.
A stock TBI 350 (RPO L05) is 200 horsepower. A Vortec 5.7L (RPO L31), or a properly rebuilt 350 with Vortec heads, can easily be in the 250+ horsepower range (the L31 was in stock form). Up to 300 horse is pretty easy. That's a 25 - 50% increase over where you are now. And you could keep your TBI with some mods.
If that's not enough, a nice 383 stroker can get you to 400 horsepower.
It's all about your goals, and how much money you want to spend.