If you're interested in building a "bastard spring pack", F350 rear springs from late 80's to early 90's have more arch than the stock Van. That's the route I went for both a little lift and slightly more spring rate. If you need more, add another spring. It's a PITA but well worth the outcome...
Your really at the mercy of the previous owner(s) as to how they maintained the engine. Best this you can do is keep up on the maintenance and hope for the best. I recently bought a '96 with 303k miles and didn't think twice about mileage. It is what it is. Cheers.
Do yourself a favor and get a low stall billet triple disc converter. Night and day difference how a low stall converter will put the power to the ground rather than feeling like a slipping clutch and going nowhere. Shift kit is a nice addition too.