Welcome to the forum. Sounds like you have a fun project. My truck looks alot like yours condition wise. Mine was a municipal truck of some sort out in Montana before it came to Minnesota, those AD trucks just seem to end up here.
Having the factory 4 speed is nice too since you won't have to shave the 3 speed off your column when you swap in your 5 speed.
Oh and you can swap to 6 lug disks easy enough. Classic parts sells a conversion kit that comes with the correct master cylender and everything. Its something like 700-800 bucks or so. Since you are going for a stock look I am sure you can find a set of weathered hood badges easy too. Gotta have those to look stock.
And.. The 216 isn't a bad motor at all if you don't rev the piss out of it all the time. What is the problem is the 4:10 gearing when combined with the low geared trans and then a motor where you don't want to run at really high RPMS. If your 216 is good and you want to stay stock check into a 3:55 ring and pinion from Patricks. That will get you to a highway cruising speed.