F-body BB subframe connector/ frame rail questions Q's

Hey guys getting ready to start the chassis build up in preparations for getting the big block back next week. And I have a basic idea on what i want to do, which is get 2"x3" .120 tubing for a set of subframe connectors and .120-.125 plate for making my own torque boxes. bigger question is could I use 1 5/8"-1 3/4" .120-.125 round tubing instead ? Which would be better, and by better I mean less affected by the twist of a big block with hard poly mounts (schumacher slant to BB swap mounts), I will be using a engine limiter, not sure if it will be a chain or solid rod, leaning more for the chain. any and all inputs are welcome. I also have to replace the front frame rails because of rot, have a nice set I clipped off behind the trans mount of a '88 5th ave, but was wondering if the 2"x3"x.120" would work for a set of front frame rails and be more rigid due to thickness than the stock factory setup,

Thanks,
-Mike

People use the rectangle because it is easier to tie into the existing rails. Round tube is ok but the weight difference is minimal.

Might want to consider replacing those rubber K member isolators with something more solid as well.

Ok sounds like rectangle it is, and I have a set of cuddazappas cnc'd aluminium isolators from a few years back, so I am good there.

-Mike

I have done both square and round connectors in my F bodies.

I used 1.75x.095 round tube run in an X under the floor. It really firmed things ups, created a natural cradle for the driveshaft, and only required minimal mods for a complete loop. However, as a race only car, exhaust clearance wasn't a problem, which in a street application would be.

Another one I ran 2x3x.083 rectanglular tube in a a more traditional fore/aft arrangement. This also resulted in considerable rigidity in the body. I welded the front plate into the front subframe, bisected the floor in the rear passenger foot well, and tied them in directly to the rear subframe. Clearance was the same as any other type of fore/aft connector. But, in using a size that large, you will have to cut the floor to keep the tube straight.

You could use this material to fab up front rails if you were so inclined. I'd say you gain some additional rigidity over using the stock rails and retaining all the OEM inner fender structure, but I couldn't quote how much. With .125 wall, you are adding a bit of weight. However, after that effort and the subframe connectors, your already a big part of the way towards a full frame arrangement.

I used the stock rubber motor mounts because I ran a small block, but I did use an engine limiter strap with heims. Never had any trouble with blown out mounts. I also had some old DC cast iron iso bushing eliminators, but Gary's are probably as solid and lighter.

Well the place I want to order from has a minimum order of $150, and a 24ft length of 2"x3"x.120 or 11 gauge would be $69, and a sheet of .120 4'x8' is $87, which I think is a bit high for sheet steel, but at $2.87 a foot for the 2"x3" I think its a good deal, now I am gonna end up fully caging the car, but i got to see what she will do in the 1/4 once I get my engine back from the machine shop next friday, btw the slant mounts on the k-member are pathetic, pretty much spot welded in place. gonna re-do them as well with the tubing with a full weld on them.

-Mike