Grizzly has had several products with the name 0704. One was a mill/drill and another was a small cnc-like lathe. Which one do you have? Anyhow, does Grizzly actually sell an upgrade package to convert the spindle and get up to 5000 RPMs? Wow...
BTW: 5000 RPMs is serious business. If this is a lathe, I hope you have dynamically balanced chucks rated for that. Also, for turning aluminum in the home-shop environment (or any other environmnet for that matter) I've been able to do everything at speeds well below what high gear can do with outstanding finish and removal rates. If you're getting poor finishes, try using coated inserts if you're using carbide and/or sharp HSS. Whatever cutting tool you use, spray with WD-40 and you'll see a day/night difference in finish. This is true for both milling and lathe work. What diameter parts are you making?
My mill stops-out at just below 2000 RPM and even with small endmills, it works on aluminum like a charm.