In your case I'm gonna go with it being a timing problem more than a fuel problem.
Since it it popping out the intake, that means the spark is likely too advanced at upper RPMs. That causes your fuel mixture to be burnt before it is suppost to be, which causes incomplete combustion, and once it gets bad enough, the combustion may occur in the intake, or head runners, which is the backfire you are hearing.
Try backing the base timing back to the stock setting of 0 degrees BTDC, and see if the backfiring goes away, along with a drop in your BLMs to a more reasonable level. You will probally notice a loss of power across most of the powerband, but if the backfiring goes away, you know it is your timing that is at fault. This is also why tuning is so important, becuase by hammering out a new spark curve, you can take full advantage of the extra power you got from running more timing where the engine could use it, but also have proper timing in the top end so you don't get incomplete combustion etc.
I realize that Brian knows waaayyy more about this stuff than I do, but just like I cannot know for sure, he can't either. Every engine runs a little different than another, and needs a chip that is built for that specific engine, by doing lots of datalogging, or dyno tuning.
What he did by having you advance the base timing was kind of cheat, while it gave you more timing in the mid range, which your engine could likely benefit from, it also advanced the timing across the entire board by 8 degrees. The ECM has no idea where you set the base timing, it always assumes you are at 0 degrees, and adds timing as the prom dictates. So, while the stock chip has a pretty conservative spark curve, and can benefit a ton from 8 degrees in some areas of the powerband, those engines do run 40 degrees of total timing at WOT, and by adding an extra 8 degrees, you are putting it at almost 50 degrees, which is quite simply too much timing. Most carbed engines never go past 38 - 40 degrees total.
All that said, with the ECM entering Power Enrich mode when you floor it can be pretty deciving too. It ignores the 02 sensor under PE, and simply pours the fuel on like crazy. You really can't have a totally accurate idea of what is happening in the top end with regards to what kind of A/F ratio you are running using the narrow band O2 sensor. This is where a wideband is almost a requirement to get your tune figured out properly. You can change the prom so that PE mode is disabled unless you put the TPS at 99%, and then still get feedback from the narrowband 02 sensor, but again, it will be a pretty foggy image of what is going on as compared to a wideband sensor.
