My most recent project purchase is this 1984 formula three LS. That is pretty much the extent to what I know about this boat, much different than my last four winns. I am about done with the interior restoration. I plan to get the exterior repainted as well. There is a mild built chevy 350 in it right now. I can't seem to find hardly any boats for sale like this and I have no idea what it is worth. Has anyone ever had one of these? What kind of cruising speed can I expect?

Welcome to OSO
I don't know anything about that model specifically, I have a 78 233 that, when it was together and running with twin 350's it would run lows 50's all day long. I have modified the hull since, but have not ran it because I sold the drive-train.
Formulas are well built, and typically have good running characteristics, but like any boat from way back then is susceptible to rot.

i had a 85, same model, went 50 gps with a 21 pitch stainless through transom exhaust. perfect lake boat.

I once had a 84 Chaparral 235 with a bone stock 350 (260 hp). Similar size and weight. Topped out around 47-48 mph. Saw 50 once. Cruised ok at 40.

If it has a 1.5 ratio drive you could run a 19 pitch. I do on an F223LS with a 383 and Alpha Gen 2. It lets the engine get up a couple hundred rpms higher, where there is more horsepower. Plus it will get out of the hole easier with a shorter pitch. But if you want to obey the manual try to keep the rpms to about 4600. That would be as OConnor said, a 21.