Help with late body on early frame

[SquareBox86]

Before I dig too deep in my latest project I though I'd ask around for some help/advice.
I have a 85 and a 86 k10 short bed trucks that I'm making one out of. Both frames were junk, i found what was said to be a late frame that was in perfect shape so I bought it. All the bracket rivets were shaved off already and the brackets were in a box. I had it blasted, gave it a quick coat and started bolting the brackets on...that's when I realized I had a problem. When I was putting on the radiator core support bracket I noticed that that it (there was only 1 in the box) was an early (pre 80) type. Measured the holes on the frame and a compared it to my 85 and 86...2 1/2" oc holes on the frame 2 3/4" oc holes on the 85 and 86 frames so right there I knew I had a early frame. I found the numbers on the frame in 2 spots and both matched: C8F343951 So if we try to decode this we get:
C8F343951
C=Chevy
8=???? Should be c or k
F= Is for engine
3=3/4 ton
4=pickup
3=1973
...so according to my research I have a 73 c or k 3/4t pick up frame...correct me if im wrong
So obviously before I go further I wanted to make sure this is going to work. What I learned online so far is that it should...with these modifications:
1) you have to cut the nose of the frame to clear the bumper
2) order body mounts for the frame NOT the cab year
...that's all I found online so far but walking around the frame I noticed these:
1) the rad core supports are different, early ones aren't available so I took the late ones I had an dropped the top holes 1/4"...now they bolt to the frame and are at the same height as the early one was
2) there are no holes for the late front brake hose brackets and it looks like the brakes go thru the frame at a different spot
3) the early uses a different e brake system, late is way easier but no holes for that either
The no holes thing is an easy fix but that's just stuff I noticed...the big question is what else am I missing like:
1) what brake and fuel lines to use
2) what gas tanks and hangers
3) what mods to the frame before it gets final coat
4) etc etc etc
Any help with what I should be checking and/or parts I should be ordering would be appreciated