The bottom end is stout. The top end flows well but a new cam will help. Dont really need any head work, they flow better than PSD heads.
The head bolts or studs are a weak point, the heads will lift above 10 or 12 psi in short order. Recently Wes or somebody floated the idea that 7.3 head gaskets were stronger than the 6.9 because of the filled coolant channel.
Soot getting in the oil can bother some, you can set these up for longer oil life with bypass filtration, and run bigger oil filters. Rebuild the oil cooler, they love to leak after a while.
Fuel was always the limiting factor for power on these, other than head lifting issues. Conestoga, R&D, and CDD all do bigger pumps. 80CC is about stock, 2.2 hp per cc to the wheels is a decent rule of thumb, so buy accordingly. You might need an electric lift pump, if your engine ever loses prime or you have fuel leak down issues the mechanical lift pump will drive you nuts.
Add a nice intercooler and make a custom intake for better flow, and get a 3 inch dow pipe and run 3-4" exhaust after that.
The old radiators are OK if you find a solid one. Otherwise you can get a cheap 3 or 4 row on ebay, or sacrifice your next child for a rodney red aluminum radiator.