The 403 engine produces a lot of torque, but don't mistake it for a high-HP engine. The block has what is known as "windowed mains" where the area around the main bearing journals is lightened to save weight. This is, unfortunately, the curse of death if you twist the engine higher than about 4,500 RPM. The block flexes and the crank breaks, and that's it. It's possible to install a reinforcing "girdle" down there, but that's not cheap.
The stock 403 heads are large chamber, and the pistons used are "dished" or scooped out to further increase the end volume and reduce the compression ratio. That was done for emission control purposes. You can raise the compression ratio by finding a set of earlier Olds 350 heads with smaller combustion chambers and swapping them out. If you run the math you can also possibly use flat-top pistons (no dish) so the CR goes a bit higher, but don't overdo it because with today's pump gas you don't really want a static compression ratio higher than 9:1.
If you search this forum you'll find that's pretty much the consensus of the 403 owners. It's also the advice given in the February 2009 issue of HPP magazine in response to similar questions from a 403 powered TA owner. The 403 is a good engine, but if you're concerned at all about future value of the car and you intend to twist its tail a bit, I'd take pat's advice above and set the original engine off to the side and get a non-numbers matching 403 block to play with. That's just my take.
