Thanks to Mike aka opellane, I acquired an almost NOS stock'ish exhaust system for my Opel GT restoration last year. I installed it this past week, with a few improvements.
My engine is fairly stock, with stock forged rods, 40 thou over forged flat top pistons, stock valves, an OGTS Combo 0.420 Hyd cam and modified cam gear w/ the Comp Cams offset bushing (set at 4 degrees advanced). I'm running a 38 DGAS on a quite modified stock intake (tented floor, welded and radiused ports and matched openings). Exhaust manifold is a Sprint, so less issues of heat transfer to the intake. I also wrapped the Spring and down-comer to reduce engine compartment temps. That might reduce the life of the down-comer, but I can accept that. The flywheel is lightened (19.4 lbs down from 23.4 lbs) with a stock clutch, and the entire rotating assembly was dynamically balanced to a tenth of a gram. I expect ~100 Hp and perhaps 110 ft-lb at the wheels. I have a Getrag 5-speed to keep the rpm at a decent limit on the highway.
The stock 1 5/8" pipe is quite restrictive, perhaps more so than the Walker muffler and Bosal resonator that came from Mike. And especially as it leaves the two-to-one down-comer collector, with multiple bends that crush the pipe a LOT. I also wanted to incorporate a wide-band AFR system to assist in carb tuning. My goal was "drive-ability", not peak performance. While I enjoy a throaty exhaust sound, the novelty wears off after a couple of hours on the highway. Driving fun places in Canada usually entails big distances and LOTS of hours....
A Calgary friend, Sterling Rempel, has an Opel GT with a similar drive-train. He bought it with an OGTS header feeding a 2" pipe, no muffler and a very nice 4-outlet ANSA resonator. It sounded great, but talking while driving was problematic. His solution, that I assisted with, was to cut in a 6" long flex joint just behind the header and stock muffler, but leave the rest as-is. It still sounds great, has loads of power, but doesn't make you want to wear noise-cancelling headphones on the highway
So unless/until I upgrade to a stroker 2.4 (and MegaSquirt etc etc). I elected a similar route, except my resonator is stock, and had all the 1 5/8" pipe replaced with 2" bent at a local shop. Cost $58. Canadian, so like 10 bucks.... Fired up the MIG welder, added a couple of meaty flanges to assist installation, and replaced the weak exhaust hangers with decent ones to keep the system close to the body.
I am a month or two from first firing of the engine, but I am confident that this exhaust system will work, and sound, terrific.
Here's some photos: