I have an 84 GSL-SE that's in need of some exhaust work.

All the cats except the small one connected to the exhaust manifold have been removed, and there's a hole in this one that's slowly gassing me to death! Thought it would be a good time to do some exhaust work before I go blind and/o r asphixiate from the fumes!!!

I was hoping to modify an old 12a header and use it to replace the exhaust manifold, and bolt it to a 12a presilencer that I had lying around (was going to use it on my 82 - 12a until I blew an apex seal)!

I was wondering if it's possible to modify the 12a presilencer and fabricate a back-pressure sensing tube like the one on a 13b presilencer? I would like to get my 5th and 6th ports working like they were intended to as they are currently disconnected.

My 12a presilencer has a "nut like" thing welded on it that looks like a tube might have been connected there at one time? Was thinking of drilling through the nut into the body of the presilencer at that point and welding a tube there, but I'm not sure what the inside of the 13b presilencer looks like (i think i read a thread that stated that the tube actually runs a short length on the inside of the presilencer?

Any info would be appreciated before I butcher things un-necessarily.