If the cat is restricted, the manifold vacuum should drop to near zero well before wide open throttle and make the truck noticeably slower than normal.
An issue that could show up on 60s (not sure about 80s) is a collapsing lower rad hose that restricts flow. A new lower hose or adding the internal spring to it solved that issue.
'03 KTM 450EXC
"If you know what you're doing, then you aren't learning anything." - A. Lincoln