Well none of that stuff other than the battery can keep a starter from cranking. And new stuff does go bad these days more often than ever before. So you could have a bad new battery.
But many other things can cause a starter to not spin as well. You said it was cranking slowly before. Was that due to an old tired batter and the cold weather? Is that why a new battery was installed?

Do you get any clicking or anything else?
What does your voltage at the battery read when you turn the key? Any change from normal?

Since you have a manual transmission. does a '91 still use a Neutral Safety Switch like an automatic does? Or is the '91 using a bypass? Or perhaps a clutch interlock switch?
Reason I ask is that you can have an NSS go bad and keep the engine from cranking. Clutch interlock switch?
In the older trucks, there was a simple jumper wire to take the place of the NSS that would have been present with an auto, and those used to go bad too sometimes.

The fender mounted starter relay can go bad as well. So you can see that you have some testing to do.

1. When you (or a helper to make it easier) turn the key to START, do you get 12v to the Red w/blue wire at the starter relay?

2. When you jump the battery cable to the small "S" post that the Red w/blue wire is connected to, do you get a click at the relay/

The answer to those questions can narrow things down greatly and send us (hopefully) down the right path.

Good luck.

Paul

 

 

Well there ya go. The basics first, then the fiddly stuff.

Glad you got it figured out.

Paul