4500 is a good shift point for the factory engine.
You might be surprised at the performance had at shifting lower. I ran my car (you can see the specs. on the link below) and shifted manually (i had a manual valve body) and experimented at different rpms, ranging from 5200-5800 rpm.
Then i entered a bracket race and left it in drive to shift by itself, which it did at around 4000-4500 rpm and my times were identical to shifting at the higher rpm.
anyway, you can get a decent AOD, built up with one piece input shaft, manual valvebody, etc...from
www.gerperformance.com for a reasonable price.
then get you a stallion converter and youll be in business.