Definitely would buy a set, especially if your prices are affordable. I've never seen them go for less than 70 bucks per size and up the larger the sizes.
When is the last time your timing chain was changed? A sloppy chain can cause all kind of problems. There's a few easy ways to tell with out removing the cover. You tube has alot of videos that show how to. Good luck gettin her goin.
I've had the same thing happen to me when I changed my brakes. I always double check any part I get now, even when it's from a parts store with a better reputation. Aint nothin worse than wasting time trying to put the wrong part on. I think I cussed for about 2 hours before I figured this can't...
Cant remember which one, but I've read a magazine article where they pieced together a 67-72 bed that was ordered. Not sure if it was LMC, Classic parts or which place, but somebody should sell all the bed pieces for 73-87 pick-ups.