Barry, when you went to Colorado you placed yourself in a higher altitude where the air is thinner, = less O2, thereby more fuel per volume of O2 = richer mix = less/no backfire.
If there is still a bit of backfiring, then after making sure the exhaust is leak free, adjust the pilot air adjusting screw for a slightly richer low range mix ...WHICH IS THE PRIMARY CONTROLLER for the backfiring issue ... not the needle. The slight effect of the needle on this issue will certainly be eliminated if the white spacer is shaved even a little. Personally, I think it should be shaved by 1/2 right off the bat. This engine is soooo freaking lean from the factory that it needs a richer mix from the get-go.
Where jetting is concerned 95%+ of backfiring is absolutely controlled by the pilot circuit.
A stock Savage engine with just a rejetted carb and higher flowing exhaust will rarely ever need more than a #55 pilot jet. With the pilot circuit tuned properly with the adjusting screw you should be able to eliminate backfirng completely... keep in mind that some
popping & crackling is normal behavior and will not hurt the engine. In fact, it is evidence that you have done the tuning job well. Be proud of your skills!
Procedurally, start with the pilot circuit first and get that jetted/tuned properly ( low range ), then do the needle/spacer adjustments ( mid range ), and wrap it up by working out the best main jet.
Remember that there is overlap by the various needles/throttle ranges, and the lower ranges do affect the higher ranges. The low range, or pilot circuit, is the foundation, so start there and build up from there.