Inside of rear sub frame

Rear sub frame opened up.
It's a slippery slope restoring these old uni-body cars. What started as an engine re-build has migrated into a full suspension rebuild, inner & outer sub frame connectors tied together to stiffen the frame, and a 4 point roll bar installation currently in process. The hoop will tie into the body right above where the rear inner/outer sub frames are welded together. The challenge to solve is where to tie in the rear roll bar supports on either side of the rear hump covering the gas tank. NHRA rules state welding a 6" plate near or around the rear coil springs or shocks is "approved." I was not satisfied with this approach, especially on the passenger side, which has a flimsy plate that extends out near the spare tire that you can actually fit your hand underneath to feel the rear sub frame. My goal was to tie directly into the rear sub frames so started cutting on the driver side to see where the sub frame was underneath the 16 gauge sheet metal. To my surprise and disappointment, I cut right into the sub frame, which is also molded in 16 gauge steel.
I decided to weld in some 1/8" 2" x 2" angle iron to make a shelf to attach a vertical 1/8" x 2"x 3" rectangular steel insert, which is welded to the same dimensional horizontal insert, which fills the gap between the lower sub frame sheet (coil is right under it) and top of sub frame where the 6 x 6 plate is normally welded to.


Horizontal bar stock with vertical plate welded into driver side rear sub frame

Vertical bar stock welded into rear sub frame on top of angle iron shelf.

Upper sub frame skin welded back around re enforced steel rectangular stock.
The final step was to weld the top skin back on and 6" x 6" plate which serves as the landing pad for the rear roll bar support.

Final product on drivers side.

Passenger side almost done.

Passenger side top skin will be welded in next, then 6" x 6" plate. Right behind the 6" x 6" plate is where you can fit your hand underneath.
The end result should be a stronger tie into the rear sub frames on each side through multiple weld points inside the frame rather than welding the plates to thin sheet metal that has a 3" gap under it to the rear springs.

 

I'm not very familiar with the rules but it might be the case that if you cut through the floor then you must go to the subframe with one continuous bar. Maybe in your case it won't be viewed that way but it might be questionable.

A 4 point roll bar won't pass tech spec. There must be a front strut.

 

if I recall.. unibody cars are to use plates that sandwich the floor pan..tubes then weld to plates..even with my subfame connectors I still use plates to sandwich the floor pan.. 2 of my plates are shaped with the sfc's under the car.to sandwith the pan....just going by there rules..

update..if you weld then only the top plate is needed..if you bolt a bar in then you teed to sandwich the floor pan with 2 plates 1 top 1 bottom then 4 bolts ea 3/8 bolt min.