Having trouble diagnosing hesitation/stumbling

One issue that crops up on smallblocks is the heat crossover plugging up w/carbon, the ports in the heads and the intake will completely be closed off. This will cause hesitation
and a choke/fast idle that takes too long to open/idle down, just an addtn'l thought. A bone stk. eng. shouldn't run poorly w/the stk. vac adv hooked up, but too cool an intake
will run lean, and the advance may cause a surge when it's pulled in as a result. I would change that t-chain jumped or not at that age & mileage anyway, good luck!