Slantzilla, you feelin' OK? That's kind of a strange answer
With the 2bbl setup, the factory used a 2-¼" headpipe all the way to the muffler, with a 1-7/8" tailpipe. Same muffler and tailpipe used on various V8-equipped A-bodies. So no, 2-$#188;" pipe isn't overkill. The pipe size doesn't significantly affect the amount of condensate in the exhaust. And where would you have him drill drain holes? Well-made mufflers already have condensate drains.
Seattle07: Your symptoms (poor mileage despite clean plugs, sluggish acceleration) can indicate exhaust restriction, and they can also indicate retarded camshaft timing. Catalytic converters, especially early ones like that on your '75, can indeed plug up. As Slantzilla says, extended rich running can greatly accelerate the process. However, a catalyst failure cannot cause a rich condition -- that's down to the carburetor's condition and calibration.
However, something else you said needs discussion: You say the car hasn't got an air pump. Without my '75 FPC in front of me, I am pretty sure that all '75s originally equipped with a catalytic converter were also originally equipped with an air pump. The aspirator type air injection (w/o pump) didn't come along until '77 or so. Air injection is a necessary part of most catalytic-equipped emission control systems, especially these early carburetor-plus-catalyst systems. Without air injection, you will have excessive raw fuel in the catalyst on a fairly regular basis, which will heat it up and melt it down, clogging your exhaust. Replacing the converter will help in the short run, but it'll melt down again. You will want to have a careful look for evidence that the car used to have an air pump. There is a fitting for an air injection pipe at the rear of the manifold mounting surface of the cylinder head, that's the "upstream" air injection point and some early systems inject air only at this point. The "downstream" point is a tube on the side of the catalytic converter. Early systems don't have it — I'm not sure at the moment what year downstream air injection first appeared. But, it greatly improves the efficiency of the converter and helps guard against converter meltdown.
So, go have a careful look — maybe show us some clear pictures of your engine compartment — and let's see what's been removed and what remains. You will likely need to add an air pump back on, with associated pipework and valves. The parts will be challenging but not impossible to find. Once we've established what the car does & doesn't have as part of the emission control system, then we can start talking about appropriate replacement converters.
As for the cam timing issue: How many miles are on the car? Has it ever had major engine work? The cams were installed in a retarded position in the '70s, and that's even assuming the assembly was done to spec, which it usually wasn't (sloppy assembly at Chrysler in '75), and also assuming no wear to the timing chain and sprockets (also a bad assumption because the nylon-over-aluminum cam sprocket does tend to wear).
You've still got a ways to go, but you're making good progress!
