I was using a 56131K, but pulled it for another truck in a pinch. Then I bought one of these (56134K):

http://www.ebay.com/itm/56134K-Tower-Solenoid-for-Western-Fisher-Plows/161561235351?_trksid=p3693.c100102.m2452&_trkparms=ao%3D1%26asc%3D20140212121249%26meid%3Dd8c9843fa7664f85bb0eeb989ce116a0%26pid%3D100102%26
It's listed as a replacement for the 56131K, and it fits the truck a lot better than the 56131K because both stock glow plug relay screws will fit the base of the 56134K.

I've got a few days on it, and it appears to be working well. I wish I knew the amperage ratings.

If this relay has been mentioned before, I'm sorry.

-Matt