GRT horsepower entry

I want to enter accurate horsepower information into the GRT display. Our 10 has a XIO540-D4A5, with 9.0 pistons, vetterman exhaust, GAMIs, stock everything else..
Stock factory horsepower is 260. Anyway to determine what we making now?
Thanks,
David

Has it ever been run on a test cell?

No it hasn't.