As I said in another thread about this:
My 75 400 tach has the 5700 RPM redline on it, and I've had the engine up to nearly 6,000 RPM before. That was when the cast rods were new. Today I wouldn't do that unless I had the next engine ready to go. Old cast rods (by definition, they're all old in 2010 since production ceased in 1978) + high RPM = high probability of coming home on a rollback.
There are 2 theoretical limits to engine RPM. One is related to the piston speed that the short block can handle. For continuous duty that limit for factory cast rods and cast pistons that maximum is 2500 feet per minute (fpm). For maximum RPM, that maximum is 4000 fpm. That works out to 4000 RPM and 6400 RPM (respectively) for the Pontiac 400. The second RPM limit is related to the speed that the main bearings will handle safely and still retain sufficient lubrication ability. That number is 60,000 inches per min for a street engine. Given the 3.000" main bearing size of the 400 CID engine, that works out to 6400 RPM. (Note that the 455 factory crank is the larger 3.250" size, and thus that same limit is reached at 5900 RPM -- lower than the 400 crank.)
Bottom line is that even with all-new guaranteed good pieces, 6400 RPM is the theoretical upper limit for peak RPM on a street engine if you are using cast rods and pistons. ANYTHING less than "perfect" pieces and preparation degrades that limit downward significantly. If you fully race-prep the engine (much stronger forged rods + pistons, oil system enhancements, etc.) you can stretch these limits upward. But doing that will get MUCH more expensive VERY quickly, and your overall long-term reliability will drop significantly.
Before anybody jumps in with the protestation that drag cars are different -- drag race engines are a different animal all together. Running an engine all-out for 6-10 seconds, then doing a complete tear down and replacement of many parts, is a LOT different that bolting an engine together that's going to run for even 5,000 miles. The presupposition here is that "street use" implies that your engine will hold together for several years and many thousand miles. Winding it up into the 6,000+ RPM range isn't conducive to accomplishing that goal.
One additional observation: Many (not all, but many) of the latent defects in cast rods can be alleviated by properly preparing them. Generally, that consists of polishing them to remove the part lines and then shot peening them to stress relieve them, and then balancing them to be sure they're all very close in weight. You have to do all of that to eliminate the stress riser points hidden all along the part lines, because that's where the stress put on the rod is concentrated and it facilitates them shattering when the limit is reached. Doing all of that isn't cheap, and it still won't cure the fact that the cast parts fail catastrophically when they DO fail. Given the cost of forged rods today, there's no economic justification for NOT using a forged rod. Yes, cast rods CAN be improved, but WHY?