422 -- The factory seats are heavy... `mine were ~50 each with the factory floor brkts... I've always weighed everything and continuously look to reduce weight.... physics= less weight=faster times (lap times, accel/decel, etc).... infact, I've gotten my car down to ~3150 w/1/4 tank fuel (w/o driver).... substitute aluminum wherever possible, etc.
I don't know how much a fiberglass hood weighs, but i know that the flat hood was 60 pounds and the twin hood scoop, hood way 78 pounds, on my70 barracuda. So you should save a lot of weight with the fiberglass hood.
I rebuilt my carbs using suggestions from ThumperDart. Went with QuickFuel billet metering blocks and light weight bowls. I don't know how much weight it dropped, but I could really feel a significant weight difference on my 850's from the stock to modified one. Will be a big weight loss since I am running 2 of them.