1987 22r lack of power

I have a 1987 Toyota pickup that I purchased a little over a year ago. When I bought it, everything was great as long as the weather was warm. When it started to get cold outside, the truck would run, but not good. It would always start and idle very smooth, even when it was 15 below zero. I would let the truck warm up, but when I tried to go, it would cut out. Some days I couldn't get the truck going any faster than 45 mph. Since then I have put on a new fuel filter, air filter, spark plugs, dist cap and rotor, plug wires, checked the coil. I have also sprayed the heck out of the carb with carb cleaner. Every time I monkey with it, it would seem to get better. I thought I had things running good, until the other day. I was driving through a canyon and I was having a hard time getting it to go faster than 40 mph. I have a 5spd. In third gear I could get up to 55 mph, but as soon as I hit 4th, I couldn't hold the speed. I knew something was wrong when I was passed by an older toyota towing a small fishing boat. Any suggestions?

1987 pup
22r
5spd
240,000 miles

Did you change the tire sizes after that summer? I jumped mine up from stock to 31 inch A/T tires and it made it so 3rd gears limit is about 45ish, and 5th is useless now.

I didn't change tire size, but I did put on some BFG ATs. But I went with the stock size becuase the guy selling me the tires told me that I would notice a difference if I went bigger.

How's your fuel economy? What do the spark plugs look like? Does your carb need to be taken apart and cleaned? Is your air filter clogged? Your exhaust may be partially clogged?

My fuel economy sucks 15mpg. I have a new muffler. And a new air filter. The carb had some work done a couple of years ago, but it was before I bought it. The spark plugs a brand new.

Speaking of clogged exhausts, whats the easiest way to clean out an exhaust pipe ruining anything?

How can you tell if your catalytic converter is no good. I am starting to wonder if that is my culprit. The truck has 240,000 miles, and I think that it is the original cat. Should I replace it with a new one, or leave it out. I don't have to worry about emmisions. I could blow smoke like crazy, but as long as my horn and lights work, they will let me on the road.