Guessing around 75/80 pounds? The steel T/A hood was confirmed at 65 pounds by NOT A TA (John@Lab14), and the fiberglass hoods were a bit heavier... They had no idea how strong Fiberglass was back then and tended to overdo it by a lot, and then some.
Firebird Central claims these springs are for the factory fiberglass Formula hood, not the lightweight replacement. You won't see that in the listing, but if you load up their part SHE-223 which is for fiberglass hoods, it says; "These are really designed for aftermarket fiberglass hoods. If you have a 70-81 original GM formula style hood, you may want to use our part number # SHE-122.".
SHE-122 is for
all 1970-1981 hoods, so, obviously the weight isn't that much of a factor above and beyond the steel T/A hood.
New hood hinge spring for your Firebird or Trans Am. Exact fit for all 1970 - 1981 models. Correct length and coil count. Replaces original GM part number 3907626 perfectly.
www.firebirdcentral.com
The information just isn't out there, probably because the people who cared about it's weight threw that sucker out! Your options seem simple enough though; buy the right springs for your hinges, guess at the weight, or weigh it and be the guy who puts the information out there for the next person.
Strut conversion seems like a lot more work than "doing it right the first time", but, that's just me I guess.
Good Luck!