I'm looking for options besides rebuilding a Chevy 2.5L straight 4 in an old mail jeep, a freind has a GM 3.0L marine engine, any ideas about whether this is a do-able swap? Thanks!

As far as I know, the GM 2.5L OHV four (the old Pontiac "Iron Duke") is the same block as the 3.0L "SuperDuty" four. I can't vouch for the bolt patterns or accessory placement on the 3.0L vs the 2.5L. Since Chevrolet didn't offer a 2.5L in the years that GM supplied parts to postal vehicles, it is my assumption that the engine you've got is the old Pontiac unit.