Frame Upgrade Question

[1964Panel]

Good Morning All,
I am proud owner of a 1964 Chevy Panel Truck (half ton)
I am planning on swapping out the complete front K member (Crossmember) with a mid 70's unit. My understanding this will be a direct bolt, then i just saw a thread saying only if they are both half ton. so is it true a 3/4 ton front will not bolt in, and if so, what is the differences?
also, is there an online reference that details the frame specs over the years.

I need to make sure i build a safe frame

Thanks all!
cb