Is it probably the ECU?

'74 Sport said
However, now it has an initial stumble and sometimes pops through the carburetor on hard acceleration from standstill.

'74 Sport said
The stumble is at low rpm from standstill. At 20 mph, kickdown does its job and it goes.

Have you checked pump shot? With the engine not running, crack the throttle quickly as if you're mashing it from a standstill while looking down the carb throat.

Do you see an INSTANT shot of gas, or is there a slight delay?