If the trans has been rebuilt at any point, they very likely used parts made for modern ATF. Even if thats not the case, modern Dex/Merc and Type F aren't that different...it should still work well enough.
Type F could help a worn out trans though, it has less additives than other ATF and will 'grip' harder.
Cobra 5.0L & AOD swap? T5z swap? C4? Powerglide? I can't decide!