69 AMX Muffler options?

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   Napa part number EXH 17732. It's a stock or OE replacement muffler (aka quiet) with an offset 2.25" inlet, centered 2.25" outlet, and the dimensions you specified. It's aluminized steel rather than stainless steel, but that's all that OE mufflers ever were. It will be very hard or impossible to find a stainless steel muffler that isn't a noisy performance muffler. Stock sound mostly means aluminized. Don't worry about restriction because each one is only handling four cylinders when it was designed to handle more than that. 
Two of those or some universal reversible turbo mufflers. Not much difference. The turbo mufflers are slightly louder but flow better. 

Run dual 2.25" pipes from your manifolds back to the mufflers, with a balance pipe between the pipes near the driveshaft front u-joint. The balance pipe makes it quieter. You can either run 2.25" from the mufflers back or use reducers and run 2". The 2" pipes will be a little quieter and easier to route but will have a higher pitched note than the 2.25" pipes. 

Add downturn tips under the back bumper. The tips blast the sound downwards towards the ground rather than straight out. It makes it quieter. 

Rubber insulated hangers throughout. Exhaust vibrates and allowing that vibration to reach the body structure (especially with unit body) will create noise inside the car. 

This exhaust system is nearly silent inside the car at all engine speeds and throttle. It's nearly silent outside the car too if you are standing in front of the car or next to it, but you will definitely hear rumble behind the car. 

Other general exhaust tricks that I did not yet mention:
Larger body mufflers are quieter than smaller body mufflers.
Noise can be reduced further by adding smaller mufflers, resonators, or glasspacks either before or after your main mufflers...if you can find space for them.
The exhaust has the highest volume as it leaves the engine. This is where flow is most critical. The exhaust contracts as it cools and is less affected by restriction later on down the exhaust system. This is why you can do 2.25" off the manifolds and step down to 2" tailpipes after the muffler without any appreciable impact on engine performance. 
Restriction isn't just based on the cross sectional area of the pipe, it's based on the length of the pipe too. You can neck down a short section of pipe without causing much restriction, but necking down a long section of pipe to that same diameter causes a lot more restriction. 