Engine pops on acceleration

[Elpresidente]

I've got an Edelbrock 600cfm (1406) carb on a newer crate 350. When I put it under harder acceleration (getting up to highway speed) that the engine will make a "pop" sound like its about to stall, then continue to run fine. Also did this when I had an 800lb load in the box, the engine didnt like the extra load. I assume it's leaning out when the engine needs more fuel, but am not sure as I can't seem to find a straight answer about this problem. It runs awesome in the power & cruise modes and at idle.
Timing is set to 10 degrees, plugs are gapped to .035" and a tan colour, choke is set to a 1/8" gap closed and opens fully vertical when warmed up. springs are stock 4"hg., not sure where to go from here. No plug wires are crossed.

Maybe this contributes to my crappy mileage as well? Guarantee its something minor i'm missing.

[bake74]

     Mine did the same thing on take off's, unfortunately I ended up taking to one of my guys that is a wizard with carb's and in 10 min. he had it running great.
     Sorry I do not know what he did, but you are on the right track.

[454Man]

Im thinking you have too much mechanical timing in your dist. Do you know how much you have?

[Elpresidente]

No, unfortunately. How would I find out?
Also forgot to mention, when I bought the carb I leaned it out 1.5 stages to a #7 on the chart (jets went from .098 -> .095, rods went from 075x047 -> 073x047)

[454Man]

Ok so it is leaned out. two options get the msd timing tape for the balancer our a dial back timing light to find how much mechanical timing you have. Im pretty sure if you retard the initial 2 or 4 degrees that would solve this problem aswell, just till you get the timing squared away

[Fairlane514]

Did you say 4 inches of vacuum?   Thats extremely low for a fairly stock motor.