My plastic/ABS headliner w/o any fabric came from LMC, fit was ok. The holes for visors were a little small, nothing a dremel couldn't fix. I used a dynamat type insulation and gorilla double stick tape, Six years later it's still up.
Why not replace the 30 amp fuse with a 10 amp or less circuit breaker? If the gauges work without tripping the breaker then you're good to go, plus it's easier and cheaper than a harness.
In order to use a 6.5TD in a square there may be 2 issues; 1) the down pipe won't fit without a body lift, and 2) the a/c on the firewall, if equipped, may be in the way.
I thought the 6.2 had slightly higher compression than the 6.5 so you need to go easy with how much boost it gets. I could...
When I've come across a one side brighter than the other issue, first thing I check are the grounds. Your results may differ, success is not guaranteed.
The 6.2 can come out by itself, unless the 700 also needs attention I'd leave it in. The last time I pulled a 6.5TD the truck needed fenders and a core support so I removed the entire nose.
My 427 & 396 have "pass Hi Perf" cast into the block, I thought all early BBC's have that casting. I'm old and could be mistaken.
The block casting number you are looking for is just behind the driver side head near the bell housing.
I had to do a lot of straightening of the pre-bent lines to get around the cross-members, fuel lines clutch linkage & front leaf spring mounts. I would only go with pre-bent again on a stripped down bare frame.
Your fuel gauge issue may not be related to needle bumping. Google "GM fuel gauge troubleshooting" and start there.
Give a man a fish, he eats for a day, teach a man to fish, he eats for life.