For a stock 91 Formula 350 TPI L98 how much more horse would it have if u rebored it to a 383 stroker?

You don't bore it to a 383, you remove the 350 crank and install a 400 crank. This and .030" over pistons give you a 383. If you don't change the pistons it will be something like a 377.

John's right, a 383 won't give you that much more hp than a 350, a little, not a lot though. What it will do is give you a bunch more torque at lower rpms, and using the same cam you'll get the power to come in at a lower rpm.