The 4.6 and 5.4 are pretty notorious for having the head gasket leak out of the back right corner (passenger side) of the motor and the leaking down. Also there is a drain hole in the firewall that will leak when the heater core gets backed up. I did the full PI head swap on my 1998 4.6 and ended up with total power gains of around 75 total if not a little bit more. That price tag was closer to about 3K after everything. If you budget is lower then you may want to just have the heads worked in a machine shop to resurface and install the typical new parts. You will need a head stud kit, all gaskets such as intake (upper and lower), exhaust manifold and new stud nuts, valve cover, IAC valve, EGR, timing chain set with tensioners, new thermostat with gasket, spark plugs (potentially look at doing new wires as well), fuel injector o-ring set (2 per injector), timing cover gasket set, pcv valve and plenty of rtv sealant. Also make sure you grab about 3 gallons of coolant (50/50 will work just fine) and oil and filter. One big thing for these is the rubber vacuum hoses tend to go bad on the IAC, brake booster and pcv valve. along with various others. Make sure you check all the rubber and if it feels bad or rock hard go ahead and replace it. I think I have it all covered. The biggest thing is to make sure that the heads gets resurfaced before installation otherwise its a good possibility you could have a leak. If you are interested in the PI head swap and have the 4.6 feel free to let me know and I can give you all of that info. If you have the 5.4 there are quite a few other options for power upgrades since you will have the motor torn apart anyways.