My beautiful and thoughtful wife got me a Bear Montana for Christmas.  I am truly blessed.  I am a novice when it comes to tuning and proper arrow spine and I want to do things the right way and get great arrow flight.  

The bow is 50# at 28" and I draw at 26 3/4" (Just measured that and I can't believe it's so short with me being 5' 10", but oh well).  I'm flexible with point weight because I don't have any money invested in broadheads yet.  

I attempted to use the spine calculator from 3 rivers but I'm getting some sort of error for the arrow input.  It's calculating 0.00.  The dynamic spine for the bow is 48.7.  

The arrows I'd like to buy are Gold Tip traditional.  I think I'm in between spines for the 1535 and 3555.  I really don't have the $ to buy both and see which fly better.  I will probably want to keep my point weight around 125-150.

35-55 should do nicely. Start full length with 150 gr tip.

I think you will find the 1535 to be a better arrow but thats just me.  Full length with 3555's with 150 grain point may be stiff.  Montanas have a small shelf and I dont believe they are cut to center. could be wrong.  I imagine a 30" or 29.5" 1535 with 150 grain point might do better for you being that you are probably getting around 46lbs out of the bow.

FWIW, i shoot 1535's out of a 46@27 whip and shoot them at 28.5 inches with 250 grain points.  Seem like it would be a weak arrow but they fly awesome and hit real hard.

Check out Big Jims sale - you can get a doz shafts for just over $40 if you don't mind the nugget purple stripe gold tips.

I think you need to get the 3555,and leave them full length. You can hot glue an insert in one of them and start to cut it down until you get the weight tip that works for you.

You need 15/35, maybe Big Jim will chime in....or you could send him a PM for advice.

600 spine will work. If you had your draw length measured while you were not shooting, it is likely shorter than that. Most everyone draws longer while posing for measurements. It's a form thing and don't get your macho in a wad over this either. It is likely to change a dozen or more times in the next ten years.
500's at 32" would be a  little more versatile but for now, I would look at a half dz or so 600s
Bigjim

I shot 35/55`s from my 45 lb bows for a while and thought they were good till I tried a 15/35. they shot much better. On the others I thought eveything was good and I just had a bad release every now and then. Truth is I would not get full back tension and the short draw was magnified with the stiff arrow with terrible arrow flight. Now with the 15/35`s it seems I have a perfect release..lol.RC