I have a 3.55 posi, with level II 2004R trans. Thinking of going up to a 3.90 I know the carrier can take the gear, the only ?question? is the axle is a '69 ZH which is a 2 pinion, can this handle a the torque of a 428??

My best guess is yes. I base this guess not on the axle, but the fact that f/g/f cars don't hook up that well, so you'll never be able to apply a huge amount of torque to the rear tires.

That's still no reason not to have a healty engine. When my 400 was fresh, I couldn't do much more than barely crack the throttle off of the line. Once you get it rolling just a tad, however, then you can really dump the coal to it.

I had a similar drive train with a 400 turbo. I also used a 200HP Nitrous kit on my 428 and the rear end held up with 3.35, 3.73 and 4.11 gears.

Was your differential 2 or 4 pinion ?

2 pinion. I put the 3.35 back in it and sold the car. It's still cruising around without a problem. You may see it sometime. It's a 69 400 car with a 'Black Rose' paint job, black standard interior. It has the YH 428, 400 turbo and the 2 pinion posi. Best 1/4 mile time at Mid Michigan Motorplex was 12.57 with street tires. I ran that one in the street legal class. TONS of fun womping the 'stangs and Cutlasses. Won a big trophy.

Pulled the axle today it is coded "ZH" but it must be a '68 it has a 4 pinion posi and it is nodular.
It has the large "N" cast in the casing. I think it will handle the 3.90 I'am putting in.