I've seen a 4.3 under the hood of a 1/2-ton. Under the hood there is not a lot of difference between the trucks so it should be very similar.
The 4.3 is shorter than a small block V-8 but just as wide. I would expect interference between the exhaust manifold and the steering, but I'm just guessing.
You might consider using a '55 to '62 truck V-8 type bellhousing because it will fit your crossmember. Doing so might put the HEI too close to the firewall though.
I haven't done this swap, but there are the first issues I think of, I hope it helps.
