The 4.3L should have a Max. RPM range of 4,400 -4,800. If you got up to 4,500 rpm at WOT, I would say you are in the ball park. In my mind 4,600 would be the sweet spot, but I've read where you want to get as close to max rpm as possible. Personally, I think the range is a bit more variable depending on your goals. If hole shot is your goal, then closer to 4,800. If cruising speed is what you're looking for, then closer to 4,400.
If you're still not getting up on plane at 4,500 rpm's there is a problem somewhere else, IMHO.
How is the condition of the bottom and drive in regards to marine growth? What is the load you're carrying (e.g full fuel, loaded with gear, coolers and passengers)?