Hi Gerry
Might not answer your question
I think the axles are different
3800 uses a 2 inch wide front leaf spring
Did The 3600 use a 1.5 inch wide leaf
But even if that’s true maybe all you need is a small spacer where the leaf mounts?
I’m gonna be in the similar but opposite situation as you when I go to fix up my 3600
My front axle is in perhaps not usable condition and I have many many 3800 trucks for parts all over the farm, I’m hoping I can use the 3800 front axle on my only 3600
-s