QUOTE(TonyAKAVW @ Aug 10 2006, 01:17 PM)
If you welded a plate across the bottom half of the firewall, and then plates over the longitudinals from the firewall up to where it meets the rear trunk, and used heavy enough material that might be one way to go. Basically box in the engine compartment with 1/4 inch plate. Kind of like a reverse Engman reinforcement kit, this time for the engine bay.
You could use the rear suspension mount points as well, also the shock towers. Between all of those spots you should be able to come up with something.
I'm welding in 1/4 inch plates at the upper end of the longs (at the trunk firewall) for my Subaru engine mount. That steel is pretty thick there, but for a whole suspension you'd definitely need more.
-Tony (not a structural engineer, nor an experienced racecar builder)
1/4 plate is WAY too thick to properly weld to the thin sheetmetal of the car. And the weight of 1/4 is a little over 10 pounds per square foot . You should use 10 gage sheetmetal which is about 1/8" and drill it for spot welds. Building a 1/4 " box around the engine is a bit too much.
But getting back to the original question I don't think you could properly add sub frames to the front and rear w/o having a cage. The connection points of tubing to sheetmetal would not be ideal. I'm not saying it can't be done, but I won't.