Up to 30psi drive pressure is considered safe. Above that, stronger springs would be prudent.

 

So if I just build some sort of sprung mechanism to regulate the pressure I should be good to go then?

 

What are you going to use as a brake? The vanes of my VNT do the job very well, but the exhaust brake works far better with a manual transmission since the auto has a non-locking torque converter.

 

I'll probably build a wastegate-style flap valve, maybe vacuum actuated through a sprung mechanism.
I sold the auto, so I'm going to go with a 5speed manual. I just need to find one that will work for me and I can afford.