318 camshaft swap 260 duration

Yes,
Hang on, I read 360 and missed the 4bbl
The 360-2bbl cam, yes; 252/260/112
The factory 318 cam was 240/248/112
The earlier 360-4bbls had the 340 cam in them and you don't want that one. IDK about 1986s.
So that is a big enough step that the drop in cylinder pressure will be very noticeable, especially with a stock TC. Keep in mind, that without changing the compression ratio,and all other things remaining unchanged; installing a bigger cam will always trade away low-rpm power to get higher rpm power. The difference in the above two cams is about 7psi in cylinder pressure,5% loss, and almost 10% loss in low-rpm performance from stall to 3000/3400(guessing). To make up for that you might need to upgrade your rearend to the next higher perf gear. Or get a new TC that stalls at a higher rpm where the torque is increased that same %.