Looking at a mild steel chassis car built earlier this year with a 12 bolt chevy rear. The car weighs 2350 with driver. Any concerns about using a motor with 850-950 HP ??? I know a 9" ford would be best. However, the car was built new with the 12 bolt.

Who built the 12 Bolt? What slick are you going to use?

Not a problem on a properly prepared 12-bolt.
I like to use the billet cap on the driver side of the carrier, and keep the backlash to a minimum on the gear set.....
No need for a 9" Phord unit....the strength difference between comparable $$$ investment units is negligable, with the 12-bolt being a more efficient unit.

I run street gears with good results in mine, but have a customer with a 2800# door car with just over 900 HP that eats a Pro gear in 600-800 passes. 8.60's / 5.40's and no troubles.

If it were a stick car, I'd probably look to a stout aftermarket Phord unit, or a Dana.

I have about 2700 passes on mine, 3250# now, and was heavier in the past.

As with any rear gears, ALWAYS pick the gear set with an odd tooth count, for better strength.
Example: Get the 4.11 gear set instead of 4.10's, the difference is the 10-41 gear count vs. the 9-37 of the 4.11 set, 9 teeth on the pinion being stonger than 10, 37 teeth on the ring gear being stronger than 41 teeth.