I fitted the MA system to my '81 four or five years back. Bolted to the stock manifolds & mufflers with no serious issues. The only problem was in getting the mufflers to sit symmetrically at the rear. Apart from tweaking the entire system to set up the muffler positioning (which is normal when fitting exhausts) I found that the tail end of the p.side pipe was a bit too long, pushing the muffler rearwards. This wasn't a problem to solve & it's better to have it too long than too short
With the stock manifolds & mufflers fitted with the duals, I noticed a big improvement in power, certainly a lot more than any other mods I've done. Didn't dyno it so have no figures, but a car that I could barely stay with when I had the stock system (with a test pipe in place of the cat) couldn't stay with me from the lights with the MA system fitted.
When going from idle to WOT it pulled well until it got towards the top end of the rev range, where it felt like it was losing power (same effect as very restrictive mufflers). This was due to the stock manifolds & later replacing them with full length Headers (plus re-jetting the secondaries) made a big improvement. NB the headers made no difference, that I could tell, at the low end of the rev range. Due to the work needed in fitting headers to the MA system, I made a custom dual system with an H pipe & sold my MA system to another 81 owner. They fit his no problem & he was also happy with the improvement.
As MA are watching this thread, here's a constructive criticism: Where the pipes run alongside each other under the x-member they can be very close together. As they're so close they can hit each other when a pothole/bump is driven over, giving a load clang. I used hose clamps around each pipe to lash up some way of holding them apart, but a couple of stainless studs (threaded) welded to the (top of the?) pipes so that a metal strap can be bolted between them would have made life a lot easier, especially as a universal rubber exhaust hanger can then be fitted between the strap and the underside of the car (I used one of the bolts holding the brake lines to the chassis). Anyway, just a suggestion.....
Yes, they could hit the deck on some bad road surfaces & speed humps, but so did the stock system. Overall I was happy with them & they fit a lot better than other "custom" exhaust systems I've fitted to cars/bikes.

 

I bought mine from Allen's in IN. They are Vette exhaust speciailists. Everything went in, fit properly, tucked up as original; inlcuded all bolts, brackets, etc. It took me about half a day. The longest part of the project was changing the 2-1-2 center mounting bracket to the new dual bracket. You have to unbolt the transmission cross member bracket and then remove the exhaust bracket and put in the new one. I put in a full s/s system with dual high flow cats, s/s mufflers and it was about $800 complete. Without the cats, you save about $400. they were real goo dto wrk with. Get on the phone, ask questions, etc. They will work up a package price for what you want and it showed up about 3 days later on my doorstep.

 

I ordered mine from Allen's $190 for the pipes and $55 for the center bracket. I will be putting them on in the next few weeks.

 

I have to agree. Way to step up to the plate MA!
I think you have convinced me to purchase your system.