I've got this Precision exhaust housing found in my parts stash. I tried emailing Precision but they never returned my email.

Hoping to build a new turbo for my 4g63T swapped Rx7.

Not 100% sure. But it looks to be for a Garrett stage 5 turbine.

Garrett used Precision Turbo exhaust housings?

No they used garret wheels for a long time. Measure the exducer diameter. That will help.

About 66.5mm, so I'm assuming allowing for tip clearance, looking at a 66mm exducer wheel

Yeah likely a 6266 or something like that. Might be a P trim though to. Not sure how loose they ran the clearance.

Likely a Garrett P-trim turbine (74/64.5mm)...they have a trim calculation of 76.

If you plan on making any amount of power with whatever turbo this will be going on, I strongly recommend going T3 instead. A Precision DSM housing for a turbine wheel that large will definitely cause reliability issues when attempting to run enough boost to make a large rotor group worth having.