Hi all, I have a 1975 K5 Blazer I've brought back from the dead after an under dash fire.
Just in case someone else has this problem I thought I'd drop a note.
I'm down to the last million or so details and just solved the speedometer problem.

I installed a new cable but speedometer didn't work. Finally noticed the entry hole at the tranny was not a square as I expected but round with provisions for a spline.
Turns out there's a small adaptor to mate the cable to the right angle gear from the tranny. I was lucky and found it laying on the ground. AFIK you can't buy this part.
Don't lose it!