Depends on how far it was driven with the headgaskets out and overheating. I would think 6 hours to do the work and 200 bucks in parts, don't know how much machining might be, I bypassed that part (which was dumb). It itsn't all that involved, not like a V6 to V6 swap, let alone to a V8.
Personally I just replaced the head gaskets and was on my way. I didn't do the valve seals, I didn't get the head machined or checked (I used a rusty rafter square to check for flatness, but didn't check for smoothness). I got the top end gasket set, there is also a simpler one, I don't think it has the valve seals and some other things. Mine was driver longer than I would have hoped "wounded", but I have 60k more miles on since then and troublefree except unrelated things, like tranny cooler line bursting, a couple of heater hoses and bypass hoses springing leaks, etc.
V6 makes an okay daily driver, I still prefer to drive the 89 with the V6 over the 97 with the 4.6, but that is because I have had the 89 for 12 years, and know every little thing about it.