318 Build? Help!

What didn't you like about the '73 360? My '72 D200 (3.54 gears and 33" tires) has a mostly-stock 360 and it has plenty of torque although the RPM range is limited, runs out of breath around 4000 RPM. I don't dislike it but then I haven't driven a truck with a 340 or Cummins.

To the OP, I like milder engines in trucks. Compression no more than 9-9.5:1 to be able to run regular gas (since mileage will be terrible no matter what), milder cam to not require a higher-stall converter which will slip and build heat when pulling loads. Better heads won't hurt but will improve performance mostly just at higher revs. With a theoretical top speed of say 75 mph (you said you don't intend to go much over 60), there isn't much need for more than about 300 HP, if that. 300 is a very doable number for a 318 with over 9:1 compression, good heads/intake (Performer RPM...?) and a realistic mild performance cam. I think those 596 heads would work fine.
I just never had any luck with 360's . Melted many pistons with a stock bottom end. Never had any that impressed me when it came to endurance. Smaller mains , Externally balanced, junk pistons, small main caps like 318 but longer stroke, That never made any sense, I would rather build a 318 then a 360. I had 408's and 416's World of difference . This is just my opinion . A stock 340 does not compare to a 360 in any vehicle. Everything is stronger in a 340 and easier to intermix with a 318 drive line. Let me know how easy it is to find a 360 flywheel or a good balanced damper for replacement in a truck or car with a 360.