thanks so much for the help
Welcome to the Team Zack!
why does it seem so hard to find parts compared to the A bodys
The B-body just isn't as popular due to it's size, but mostly it's weight. There are two ways to go fast. One way is to have a really big motor, the other way is to loose weight.
Look on U-tube video for the 5 O.M.G. where two guys pulled a Mustang 5.0 Fox body motor and put it in an MG-B convertible that was a 1000 pounds lighter. It scared them with the increased power to weight. Anther example is a 500 cube full size Cadillac that Hot Rod magazine kept cutting bits of the body off with a torch until it was down to just the frame rails. They tested it and showed how much quicker it was as different parts where cut off.
You can buy a brand new Camaro (2,860 pounds), or a Tri-Five Chevy (3,100 pounds) which are popular and have a lot of demand for parts compared to a 1971-'74 Impala that tipped the scales at 4,530 pounds).
i am going to have to replace my rear deck filler panel as it is rusted through, but i cant seem to find one. should i just fabricate my own
Probably but I would check with Classic Chevy and the used car lots in the desert south west that have rust free cars for parts first.
the rain guard trim above the doors has caused rust, ive heard of people cutting them out and grinding them down so its flush with the body, seemed to easy, wondering what would be best
That is a mater of personal preference. It is called shaving and people remove drip lines, door handles trim and other parts to get a clean look. Personally I only take my cars out on dry warm days because I am traction limited. So a drip rail wouldn't bother me. It is there to keep rain water out of the car that runs off the roof.
i blew the rear end up and thought about just getting a whole new rear end because it has a small 10 bolt, is there any rear ends that would be better then others
Hard to do with a TH350 (that isn't that strong) and a 350 that can be a great power plant; but not likely to break parts in stock trim. Now if we are backing the car up and throwing it in first to break the tires loose then yes you can bust a rear end. The Spicer ten bolt is as strong as most nine inch Ford rear ends that share the ten bolts 28 spline axles.
I broke rear ends, transmissions, drive shafts, universal joints and clutch parts all the time when drag racing with a high horse big block. I finally got tired of rebuilding 12 bolts and just install a Dana 60 under all of the cars that I build as part of the car build.