SC power question
 

I was wonder how much HP a SC chassis can stand before flex and damage to body starts

Coupe?

Shoot, targas and cabs flex with the stock motor. in a good way... ha

The turbos do OK with a 915.

There's probably not a definitive answer.

People put Chevy V8's in the back, hanging off the 915 tranny...

First, its torque not power that will flex the frame. Generally if you want power you get more torque, but there's always the option of going for high revs and 'modest' torque.

Coupe or targa?

Any reinforcement (e.g. strut tower brace)? Any rust?

I'm not sure what the answer is, but I know its more than the 915 can take, even with modification, so if you have the stock transmission then it will break first. I'm fairly confident that the Coupe can handle the max rating of a G50... but if you are going to the effort of putting in a G50 why not get a Carrera instead?