355 surging a little at steady rpm on highway

I just installed a 355 in my 84 monte carlo but i took out the ecm and am currently using a 1976 vacume advance distributor. when i go at a steady 2500rpms (55mph) my car seems to surge back and forth just a tiny bit. like it needs either gas or more air idk? im also thinking it may be the distributor. i replaced the weights already. i have already hooked a vacume gauge to the carb to make sure the jets are tuned to the highest vacume. there are no vacume leaks. is it possible to test the vacume advance with a gauge?? i dont have a timing light.HELP ME Why is it surging?????

Try stepping up jet size by one number.
Tuning for highest vacuum generally puts the Air Fuel Ratio on the lean edge.
It's a good reference point to know where the lean base is and then go up one or two numbers.
Then you can go back to playing with the distributor for best performance.
Consider looking into a cheap timing light. It doesn't have to be ultra trick, just a light that will put you in the ball park.