it depends. the truck engines (both the 4.6 and the 5.4) are NPI and PI, same as our cars, but the years are a little different. also, some trucks (not all) have PI heads, but they still retain the NPI intake ports. meaning basically that the combustion chamber is smaller on the PI as compared to the NPI truck heads, but the intake ports are the same, NPI or PI (keeping the NPI design). i remember reading somewhere that this was for torque purposes.
but to answer your question, like JR said, yes, 5.4L heads will fit on the 4.6L block so long as you use the 4.6L intake manifold (and exhaust manifolds).
is it worth it? only if they are the PI heads that have the PI intake ports. if not.....then no, not really.