Really bogging down in 3rd gear

- Auto trans? 3 speed?
- Is the car revved up a lot when it shifts, or just normal RPM's and speeds and cruising?
- Does it run better in 3rd when you finally get it revved back up?
- Does it bog after shifting from 1st to 2nd?
- Is the timing setting at idle per the timing cover tab, and is timing set with the vacuum advance hose removed from the distributor and plugged, and using a timing light?
- Is this an older points distributor?

I would be thinking carb's power valve not working (you have a Holley 1920, right?), ignition timing (perhaps mechanical or vacuum advance not right), overall timing (basic ignition timing, or timing chain, but not likely just the chain as it would be doggy off idle in 1st and 2nd too), or fuel restriction. Answers to the set of questions above will help narrow this down.