No power, runs rough when shifted into drive

Is the idle speed too high or too low? It sounds like you are describing a vacuum leak. You wont get any fuel to go with the air that sneaks in past the carburetor. The idle speed will be too high but without a corresponding increase in power. I would start by pinching off the vacuum hoses. Next, if you have an EGR valve, that could be sticking open due to carbon particles. Loosen it and stick a metal shim in there to block the port. If the running problem clears up, clean that valve or replace it.

Dandy. Look for the vacuum leak. Pinch hoses, or spray water over the engine while it is still cold to see where it gets sucked in or where it makes the engine slow down momentarily. Spray water on the carburetor's throttle shaft bushings and its base gasket. That gasket is about 3/8" thick. There are small channels cut into the base of the carburetor and it does not take much for the gasket to deteriorate and leak. Obviously be sure there is no vacuum ports on the side of the carburetor that are not plugged or being used. I used to race 318s and 340s, and I always had custom projects. That means every car had at least one port plugged off with a short piece of hose and a drill bit. You need drill bits to go fast!

What state are you in that you still have a 1975 on the road? I am searching for a rust-free 1988 Grand Caravan with 15" wheels to replace my rusty trusty van.

Holy cow. I am in the middle of Wisconsin. My best friend's son has been working for Boeing in Seattle for about five years!

Since there is no power, are you sure there is no obstruction in the exhaust?

One other thought. Since it fails to accelerate quickly, are you certain that the accelerator pump is working properly? Ignition timing?