Paul, Are the wiper arm transmissions (the splined part the wiper arms attach to) still the original 1950 units or did they get changed along with the wiper motor? In reading up on the arms for your 1950 Burb, originals were Left: 11-15/16" long chrome GM# 3697281 and Right: 11-15/16" long chrome GM# 3697282. Those are both fixed lengths (non-adjustable).
Chevy lists an alternate GM# 3703494 - Arm, windshield wiper assy. (universal) (adjustable 9-3/4 to 12-1/4) that fits 1949-56 trucks (in the 1929-57 Chevy Master Parts book). You can find NOS ones
like this on eBay or you can select from the various reproductions offered with vendors like
those listed on this page. Hope this proves helpful.
