The barrel length does not have a lot do to with powder burnup.
Too slow a powder will leave unburnt granules behind in an 8" barrel... or a 28" barrel. I get unburnt IMR 4227 in my 6.5" .44 magnum, but the load shoots very accurately.
I doubt you will get a nice clean burn of N350 at .38 Special chamber pressures, even at the maximum. It is more suited to heavy bullet 9mm, .38 Super, or low end .357 Magnum.
I realize you are trying to do it all with one powder because of high component prices and that is just one of the results of such a compromise.
If you get decent accuracy, don't worry about it. Just point the barrel straight up when you eject the empty cases so as to not get unburnt powder granules under the revolver's extractor.