The upper frame member of the welded on rigid rear looks a little flimsy to me. It's in compression and it's quite long, so could collapse if enough torque were applied at the rear. Also, it's not in line with the upper frame members. I suppose if it held up to drag racing, it should be strong enough, but who knows if it ever really was raced? I'd be a little cautious and beef it up as I bet you're going to have a bit more torque than a stocker!
Tom