Mexican Exhaust huge restriction

I bought a used late moded Mexican exhaust system to play with as my '04 Mexi is on a '65 tunnel I don't need an emissions test. My other '04 is on a 70 chassis and it needs emissions test but it too doesn't need the cat converter. My thought was to gut the cat and try the muffler on one of them and see what kind of difference ther would be in performance & MPG.

Was I supprised to see what the cross over / connector pipe 1/2 of the exhaust system looks like internally !  Where the #1 cylinder (short) pipe that connects to the heat exchanger enters the main pipe it looks like the short pipes were just welded over a solid pipe and then drilled out. There is only a 3/4" at max oval opening. I can only assume where the other three pipes are welded on it is the same story.

This HAS to be terribly restrictive, not to mention that the pipes are not equal legnth. I was going to drill out all the excess flash and give it a try with the gutted cat but but I'd have to cut up the whole exhaust to get at the other three connections so I may go back to my original idea.

A stock Germann muffler is actually a well thought out piece of work. It has two pipes internally that make cyl's #2 & #4 exhaust the same legnth as the heat exchangers. I'm goint to take a 66 - 71 VW Bus muffler ( one tailpipe on the left hand side ) and remove the whole heat riser system then weld an oxygen sensor bung to the site where the heat riser used to exit behind the tailpipe.  The Mexi tail pipe is 1-1/2" the German muffler tail pipe flange is 1-1/4".  I don't know id that would make a big difference seeing that I'm opening up all the other restrictions.

Does anyone think I should cut the small tailpipe flange off ? I can weld in a larger tailpipe right to the muffler can.

I know this is a lot of work but I want the car to remain STOCK looking with the single tailpipe.