S-10 5 spd. transmission swaps?

I've got a question to you guys who know more than I do about swapping GM transmissions. I have a 1981 El Camino SS with a close ratio 4 spd. trans. My wife and I gave the car to our son who drives it to/from college about 350 mile trip, and we're looking for the LEAST expensive option for installing a 5 speed into it for better mileage. I'm not concerned with the transmission's ability to handle high horsepower, as the factory engine in the El Camino is only rated at about 250 hp, and that's WITH the aluminum heads, 4 bbl carb and intake, Thumper roller cam, gear drive, headers, and dual exhaust. There's only so much hp to wring out of a 305, and I don't care what you do, short of bottle feeding it. LOL. So handling the power of a V-8 isn't an issue here.

I can get 5 spd. trannys from S-10 2.2 4 cyl. trucks for under $200 all day long. And about the same for the V-6 tranny, which I understand is the SAME tranny. Isn't the New Process 5 spd. used in the S-10 trucks the same New Process 5 spd. used in 1/2 ton trucks with the 3.6L V-6 and 305-350 V-8 engines? I have seen these trannys that bolt to the bellhousing with 4 bolts like the saginaw and Muncie trannys, and I've seen units that look like they have an integrated bell housing. If this IS an integrated bell housing unit, is there an adapter that will allow me to adapt the S-10 tranny to a V-8, as would be needed if swapping a V-8 into an S-10 without changing the trans? I could use the same adapter to adapt the S-10 trans for use in my son's El Camino. I'm looking at this option, because everyone seems to think the T-5 and T-56 is made of GOLD and want over a grand for a tranny with over 150K miles on it. Mike can't afford to pay that for a tranny that's only going to live in the car for a couple of years. I'm installing an LS-7 and 4L60E in the car for his graduation present, so I only need to give the kid a little better mileage with the overdrive gear at a price HE can afford to pay. Any help would be appreciated. And yes, I could afford to buy a new 5 spd., 6 spd., or a Gear Vendors Over/Under Drive unit. But that isn't the point. I'M not buying the transmission. My son is, and he's a struggling college student. And it wouldn't teach him anything if Daddy came to the rescue any time he needed or wanted anything. I'll help him research it, and I'll help him install it. I gave him the car, and I pay his insurance. The rest is up to him. So we appreciate any help you can give us on making this happen for him.