5.9 Magnum. *Basically* bolts in place of your 318. Uses a roller cam, fuel injection right from the factory. Good performer as is, very responsive to basic mods. Still cheap and easy to come by in the boneyards, and are generally in better shape than LAs, requiring less $$ to freshen up. The electronics adapt in reasonably easily. "Bolts up" to your 904.
Checks all the boxes, meets all the criteria.
The 904 is a good trans. and will serve you well after being freshened up, but with your milder combo consider a 32rh, which is basically a 904 with extra clutches, lower gears and a lockup convertor (AKA a999). Not quite an overdrive, but the lockup does help on the highway; and it BOLTS IN WITH NO MODS NEEDED, as opposed to the overdrives which require surgery... just a thought.
8 3/4s are getting pretty pricey, 8 1/4s are a reasonable alternative and if it came from an A body, it's a bolt-in. Drawback: if you want to stay with your small bolt circle, A body 8 1/4s only came with the 4.5" B.C.
P.S. Nice lookin' car!
