I would DEFINATELY use the 4500 in the K10 450.
*edit, depends on 2wd or 4wd NV4500, but even with 2wd, I'd divorce the transfer case in a '78. Should be able to trade your married for a divorced.
The 4500 is probably overkill for the C-10.
So, I'd still put in the TH350, and use a cable linkage on the hump.
1. It's easy enough to patch up a hump
2. The th350 is plenty of tranny for the truck
3. The th350 cable shifter can be used in anything (except the civic
), so it's not a waste of money
4. I said so