Another slow 340

That thing should trap well over 100 mph, I'd start with compression/leakdown tests, if it's popping you might have 1 or 2 bad/sticking valves,broken/misadjusted rocker arm, bad lifter, bent pushrod? an engine scanner would help tell you if the ignition is doing it's job too. The 3.23 gears aren't helping much either. Good luck!