This is just a real rough guess,Indian Archery had broadheads in 1949.
To obtain the correct string length measure from nock groove to nock along the limb curve on the belly side of the bow(side facing you with bow in shooting position).
This is the bow length. if you subtract 4" you will be real close to the correct string.
You also need the know the bows draw weight as this will determine the number of strands in the string, ie; 60#=16,50#=14, 40#=12 these are guideline numbers. Strands can vary to accommodate nocks, noise, wear etc.
If you still have trouble with a string email me and we'll see what else we can come up with.