I know that on my fj60, the higher the voltage my alternator puts out the faster my wipers run. It's quite a big difference in speed between 13.5v and 14.5v. My reg was failing is how I found out. I don't know much about 40's, but if you are so lucky as to have an adjustable voltage regulator, perhaps this could help. I'm told that at much over 15 volts you can boil the fluid in the battery, so be careful if you go this route.
The wipers on my 68 were slow also.
The following things all helped:
Cleaned electrical connections in plug-in connectors. I had a bit of voltage drop across them.
Checked and lubed the bushingg in the windshield frame, They were dry and a bit sticky
Cleaned good out of cable between left and right sides and lubed with some lubriplate.
Took motor apart, cleaned bushings, replaced brushes and lubed bushings.
Took the driver's side gearbox apart, cleaned and relubed.
The first 3 things helped the most and were the easiest.
Fresh blades help too.