Truck has started running rich & terrible MPG

[ih8vols]

Had my '79 sidestep for just over a year now. Drove it about 3 hours home when I bought it and it did great. No gas mileage issues at all.

Here's my symptoms:

• Over the past couple months, I've started noticing it's running rich. I can smell it in the tailpipe. Something else I've noticed is that it usually starts decently. However, if i drive it long enough to warm up, and then stop and get gas, when I got to start it, it STRUGGLES to start. Almost sounds like it might be flooded.
• It is GUZZLING gas right now
• There's a cheap tachometer installed in it. Idle is right at 1170. In gear, it's at about 1650. Yesterday, I took it on the interstate and it hit 5000 rpms when I hit 65mph
• The carb is VERY CLEAN. Previous owner had it rebuilt right before she sold it.

I did not grow up working on cars. I actually bought this truck to begin learning how to fix them.

The engine is the original 350. The carb is a Rochester Quadjet.

I bought a vacuum gauge today, but haven't hooked it up yet. Plan to do that tomorrow.

From what I've read, it appears the carb could need tuning, the ignition timing could be off, or maybe a vacuum leak?

Based on the symptoms I'm experiencing, what would you recommend I check first? The symptoms popping up so quickly after running great for so long is what I'm struggling with right now.

Appreciate your help!