If you ever plan to add boost in the future, and for general longevity, I'd look for a low mile engine that was pre-1992. You'll get forged pistons in "most" pre-92 302's from Mustangs, Thunderbirds, etc.. 1987-1992 anyway. I'd also look for an engine that had (at the very least) E7 heads. Explorer, GT40 heads would flow even better. Avoid 1970's stuff if you can. It will flow like shit and be down on power. E7 heads are a pretty nice cylinder head for a factory casting, but as I said, the GT40 stuff will flow a bit better and make more power.
I went with a T5Z transmission from Modern Driveline. A perfect match to the truck. Fit's well and hooks up the the factory Ranger clutch master cylinder with an adapter and a -4AN hose. I did use an aftermarket Tilton slave cylinder in the bellhousing though. There are lots of cheaper options for clutch slave cylinders, but most take more fabrication work and troubleshooting. In my opinion the T5 shifts are far superior to all other manual transmissions that can be used in this application. Mine shifts like butter. Very quickly. The M5OD won't do that. Neither will the ZF.
You can find used T5 transmissions at salvage yards in Mustangs. But for the cost of one, I'd just search out a local T5 rebuilder and buy one from them. There are lots of guys all across the country rebuilding T5's and most of their prices are pretty decent considering you're getting a tranny that "should" give you less problems than a used, unknown condition junkyard transmission. You can also rebuild one yourself as they're dead simple transmissions. Avoid the 4 cylinder Mustang T5 transmissions at all costs, as well as the S-10 model of T5. I hear 94-95 Mustang T5's are nice and strong units, but if you grab one from a 94-95 Mustang, be sure to grab the bell housing from it as well, because the input shaft on those transmissions are a little longer and require the 94-95 SN95 Mustang bellhousing.
If you're worried about the shifter coming through the original hole in the floor, then that's another issue altogether. Modern Driveline makes a front shift/rear socket tail housing for the T5 transmission that puts the shifter in the stock location, but most people won't buy them because of the (sticker shock) price. I used one and couldn't be happier with it. A Chevrolet S-10 T5 tail housing can be adapted, but it will likely give you long-term oiling issues internally on the transmission, and it can't really be modified to completely correct the oiling issue. I talked to several professional T5 builders about this issue and they advised against using the S-10 tail housing. You also have to have one from a specific year of S-10 to get the right speedometer drive location, and by the time you find one of those, they're priced pretty high normally, so the Modern Driveline tail housing starts to make much more sense as far as cost is concerned. It'a bolt-on affair with the correct speedometer drive. The MDL front shift tail housing also accepts performance shifters (depending on if it's a front or rear socket version - call MDL and ask them about that - front shift, rear socket takes a Mustang II aftermarket shifter, while the front shift, front socket uses any aftermarket T5 performance shifter), unlike the S-10 tail housing which really limits you to only one aftermarket shifter or the crappy S-10 factory shifter.
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