Hard pass. That's a $1500 truck to me. It's been beat to death ALL it's life, been screwed with and not maintained most likely. The plexi back glass is the perfect example of that. I'd be curious if there's damage to the cab back there, may not even be able to put a window back in it without serious repair work. The Dana 60 instead of the 14bolt is very questionable. Why? What happened to the 14-bolt? Or does the guy not know what he's talking about and it still has it's 14-bolt?
Doors are sagging, could just need hinge pins but if they've been that way long enough, it'll need full hinges replaced which is a huge PITA. Gauges have been replaced? So who knows if it's truly 300k or if it's more? That'll be a rubber floor/roll up window truck, for good or bad whatever your opinion is (bad to me, I want power).
Jasper transmissions are iffy whether they're good or not, my dad hates them cuz the county kept buying them for the fleet trucks and his guys were R&R'ing them constantly. That 454's gonna be good for 8-10mpg too.
I'd much rather throw in a couple more grand and find a better truck IMHO