23’ Corsair 23 degree deadrise at transom
3900lbs.
500hp (installing soon).
Bravo 1 w/ 1.5 ratio.

I have a 21p stainless Mercury 3 blade that came with the B1 drive.
I bought this boat with no engine or drive and have no starting point.
This hull goes 58-60 with a stock 330 454.
Hoping for high 60’s with the new engine.
My engine is a hp500 clone with a slightly hotter cam and better heads.

stock X?? borrow a 23 and a 25 mirage 3 blade.... that should get you in the ballpark and go from there...they are both popular sizes/props so it shouldnt be hard to borrow one...

Is X the name of a propeller?
also I’m still new to this stuff and I see there are different diameters available for these mirage 23 and 25p props. Is there a recommendation on diameter? How much clearance does the bravo 1 allow for?

The "X" he was talking about X dimension which relates to how deep the propshaft is below the bottom.
You won't have any prop clearance issues with any of listed diameters. I'd start with a 14 5/8-14 3/4 Mirage Plus in a 23 pitch or maybe a 25.