[alchemy]

The gap at the rear mount hole in the subframe looks about right. The firewall welting pad is supposed to be slightly thicker than the welting that runs under the rest of the subframe. So other than some minor shimming needed, I think your problem is in the outer sheetmetal, not the subframe shape. Maybe all the outer sheetmetal needs to be tilted down towards the back.

That sounds like a ton of work. I highly suggest fitting a fender to the body and frame before going any further. If the car is a hot rod, the extra work might not be worth the squeeze.