Insufficient data. Did you do the work yourself? What type of valve springs were used? Was anything else done to the bottom end of the engine (is static compression factory)? What all was done to the TH350 trans? Popping out of carb could also indicate valve float if the stock springs were retained. The TA 212-350 needs TA's Stage 1 springs. Put more info here and we'll be better able to address your issue.
I think perhaps we have found your issue. Original springs were weak new, and now they're over 40 years old. Aftermarket cams tend to have more aggressive ramp profiles, which will run fine cruising on weaker springs, but once the RPMs begin to climb, aren't stiff enough to hold the valvetrain in a stable condition. This will cause the valves to stay open even after they're supposed to be closed, which can cause severe intake reversion and if it's still open when the spark fires, will ignite the fuel/air charge inside the intake manifold. Not saying this is what's happening, but with original springs used with a TA 212-350 cam, it is certainly a strong possibility. You say 2-3 shift hangs which allows the engine to rev up, and this is when the popping can be heard. Does the engine seem to skip/run rough after this for a short time? I've done my share of valve floating when I was in my younger years, but only on the big blocks. My stock 350's didn't have this issue (but I used the stock cams, which the springs are just fine for).
Regardless, popping sounds don't normally come from your transmission unless there's something seriously wrong going on inside it (been there too, lol). May I recommend creating a post in the Small Block Tech section of this website so that this issue could be addressed by the techs who frequent there?
Stock springs will work with that cam, yes. They're not optimal, however, and for best performance, the TA Stage 1 springs are recommended. Heck, even new stock springs would be better than what you have now, and you can adjust the height on them to give better tension. Again--not saying this is what it is. Just a possibility. Put a post in the Small Block Tech forum for more input. Best wishes to you!
