I missed this question until just now. So..................The first thing I always do is throw a lot more lead in the timing. Our big blocks respond to early advance like no other engine I have ever worked on. With 500", 3.55 gears and 2,500 converter, I would put in as much initial as the starter will pull. 25* or more as long as it will start hot. And then, get the rest in by 2,000 +/-. This is more effective the bigger the cam, but is always a good idea even on nearly stock motors. Even if that turns out to not be a big part of your problem, it will still pull the lower RPMs much better. And might just do the trick for your bog, too.

The stock curve is a very conservative one to allow for less talented operators to be less likely to hurt something. Much of these kind of things were designed with warranty and engine safety in mind, more so than peak performance. The stock curves are ALWAYS too conservative. Just like most cruise ratios are too fat. Safety first.

Only after getting the ignition curve dialed in, should you start opening up the carb. And just an FYI, the transitions in the carb are the absolute most difficult area to get right. Mainly because you have as many as 3 circuits coming and going at the same time. Even if they were all in the right range of mixture, their timing to each other could be off enough to cause a problem. Usually, if a carb has real problems in the transitions, it is tough for the typical guy to get right. Which is one of the main reasons so many guys pin their hopes on plug and pray EFI. They expect it to tune itself. And sometimes it does.

The key to picking the right carb out of the many, many choices out there is to get one with a tune in it that is as close as possible to being right for the particular combo it's going on. That way, a guy doesn't have as far to go to get it right. The farther away a carb is from being good with the combo it's going on, the more changes and circuits that will have to be experimented with. Just look at the varied suggestions in this thread already.

Ideally, a guy needs to determine his CFM desires and them try several carbs with different calibrations in them to see what his combo likes. Street calibration, race calibration, in between, vacuum sec., mech. sec., what boosters does it like, and on and on...................... Which is part of the reason I suggested borrowing another known good carb. If it does better, you have a better handle on the tune that the combo likes. And buying another carb (as long as it works) can be a bunch better than trying to re-calibrate a carb that has the wrong tune to start with.

I guess other than suggesting starting with the timing curve, the other point I would like to make is it is much, much better for most guys to find the carb closest to what his combo likes, rather than try to re-calibrate a carb with the wrong tune.

That's what I think anyway.