engine performance

Yong,
The 750 double pumper carb is probably way too big for a stock engine. Try a 600 Holley or at the high end a 650 double pumper. The 750 is probably causing the car to bog down through the rpm range.

Assuming the electronic distributor has vacuum advance, not just centrifugal, it sounds like too much carb. A vacuum secondary carb works MUCH better on near-stock engines than a racing type double-pumper. DPs really need to be set up & adjusted on a dynomometer, or they invariably run very rich & behave as you found. A 600cfm vac-sec carb will give good around-town performance and maybe 20mpg on the road, but will be restrictive at engine speeds over 6000 rpms. A 750 vac-sec carb will give 30 more horsepower with no other changes, will drop fuel mileage to around 14-16 mpg, and will allow speeds over 7000 rpm, which will probably destroy your stock engine without lots more special parts! I recommend trying to swap the 750DP for a 600(ideally) or 750 (alternatively) vacuum-secondary type carb. And add a rev-limiter to your distributor if you choose the 750!

is the firing order correct 13726548, remember to keep 6 and 5 at oppisite ends of the loom to avoid cross fire.