The problem started a few months ago and I haven't really driven it since, I'd do start it up once and awhile though. The problem is the truck hits 35 MPH and won't go any higher I checked the tyranny fluid and didn't see any problem but my teacher (I go to a tech school for automotive) and he told me its possible for the car to not go any higher than 35 if the catalytic converter is clogged. Now I'd rather not rip the cat off its a doomed project but probably will by the end of the night so if you have any better suggestions that'd be great. I love this truck but if its the transmission I can't keep it so whatever you got if it works it helps a whole lot. Thank you
The engine is a 4.3 l v6 Tbi, 700r4 trans, ran fine before a day before it was slightly missing at 35 but could go to 45, once it hits 35 it will over rev.

 

usually if its a clogged cat the truck will just be slow all the way around. i had a s10 with a clogged cat and it would do highway speeds but once you got to like 45mph it would take a second or more to go each mph after that. so its basically like having a 20hp truck

from the way your describing it as soon as it hits like 40ish the engine will rev up? thats usually caused my overdrive being shot. if you dont want to fix the transmission put it in D instead of OD and just drive it. my 93 i bought about 4 years ago and overdrive was shot, im still using the same transmission to this day