Your tranny is a good solid truck tranny and as good as any for what you want to do(SM420 or SM465 most likely). You also have the gears you want for working. If hauling cars or dirt is what you want to do, I agree with most here that a 292 is the way to go. Your top speed won't change but the loads will feel much lighter. I'm not a mechanic but I believe it would be the easiest and most cost effective way to go as I'm pretty sure the 292 mounts to the chassis and the tranny same as your 250. For a little extra oomph, look for one that already has split manifold and upgraded intake/carb(s). Check out Ebay and inliners.org. Be patient. They are out there and they are cheap.
My neighbor has a 292 w/SM465 in an 80's 3/4 ton. It's a torque beast. I mooch using it sometimes for hauling and it's a fantantic work truck, highway or city.
