Everything happens for a reason. The first asshat did you a favor. I think this other one with the 283 and the OD unit is a better looking, better condition truck too. Hopefully that deal works out. That'll be the **** having 4.56 gears in the old drop out rear axle, but an OD unit to compensate and can still drive it on the freeway. And 3 on the tree too. Hell yeah. And those 283's are pretty good on mpg for what they are. I actually got better mpg in a 67 with a 283 than I did with a I6 230 that I had in it, not to mention more power and it sounded much better too. It would be cool to have the I6, but really, a good old 283 with that little 2bbl and power pack heads is all as reliable as the I6.
The 283 also loves loves loves rpm. They are small journal small blocks and come with forged cranks, so they'll spin the rpm up pretty quick and cranks are strong enough to take the abuse. When it comes to build it, you can bore it out, .030 .040. .060 and even safely up to .090. The earlier 283's like in the early 60's you could even get them safely to bore .120 (1/8 inch). I know of at least 2 283's that were even bored .125 which took it out the same bore as a 327/350. The guys used OEM flat top 327 pistons from a small journal 327 that they bored .060 and repurposed the OEM 327 pistons in a 283 for a cheap rebuild job that was all as good as a brand new motor. Here's the catch 22 though, the early 283 blocks that would take the bore out to the same as 327, to fit a 327 crank in it and make it a 327, you'd have to grind out part of the block for the bigger 327 crank throws. The 283's that would only go .060 or .090 would take the 327 crank without a problem. So if anyone has ever wondered what made the Chevy 302cu in, it's a 283 crank in a 327 block. These guys that did bore a 283 out to 4.0, that's exactly what they had then was a 302. A little RV cam, nice intake 4bbl carb, a little port work on the Power Pack heads or a set of double hump heads, and headers, you'd have an ass raping little motor that turned a ton of rpms and begged for more. And how did the 307 come about? It's a stroked 283 with a 327 crank. Of course by the time the 307 came out in GM standard production, it was a large journal motor, but guys were already making 307s before GM did by putting 327 small journal cranks in 283's. The GM 307 was getting smog riddled and compression cut down so it was a gutless wonder, but the homemade 307s from small journal motors were quite peppy and torquey for what they were.
However, I went down a rat hole with that conversation as usual, but point is, for this truck, I'd keep that hard to kill 283 in it just as it is stock with the Power Pack heads, the 3 speed and the OD. That's a great set up for an old truck.
That OD unit alone is probaby in the $3000 range if bought new. I just love the idea of keeping that 283 3 speed column shift with 4.56's in that truck leaving it just the way it was back in the 60's, but yet having the OD unit bringing into modern day driveability at 65-70mph. Hopefully you end up with it. I'm sure you'll enjoy the hell out of it, and I'll sure enjoy watching what you do with it. 
