A 2nd gen,..k5(73-91 yrs), and a 2nd gen k10, & k20(73-87 yrs)=same spring perch width,...iirc,..its 42.5''... if it fits a 2nd gen k5, it should be a direct bolt in swap to your 77 k10's rear springs(as long as rearend is for a 2nd gen body style k5, not a 1st gen) ,and you said your k10 is a 77 model, so it is a 2nd gen body style...the 14bff rearend must have come out of a 2nd gen k20, or had the perchs moved, either way,... again, if it fits a 73-91/2nd gen k5, it should also bolt up to a 73-87/2nd gen, k10 or k20 truck!
the 2nd gen, k30 has a different spring pad width,..(iirc, its 40.5'') and is the same perch width as a 69-72/1st gen k5 and the perches would need to be moved to fit a 2nd gen k5,k10, or k20,... this only applies to the rearend,... the front is the same spring perch width for both 1st & 2nd gen, as you stated..but,they have different length leaf springs in front end between 1st & 2nd gen, but both have the same spring width up front! ...this is how i remember it explained to me...if i'm wrong, someone will chime in..., but to recap a 2nd gen/73-87 k10, k20, & a 2nd gen/73-91 k5 have same width rear springs, so if you mean it fits a 2nd gen k5/blazer, then yes it will bolt up to your 77 k10
if the rearend is for a k30 w/perches moved for 2nd gen/73-91yrs k5/blazer, it will also bolt into your 77 k10! you didn't mention what year blazer, so i wanted to cover the whole story! hope it helps!
