I have a 98 f150 4.6. After having alot of issues with cylinder 8 and not being able to fix the code. I went through plugs wires coils. After a compression test it was releaved that we have some internal problems. Probably a cracked valve as the spark plug was found damaged in orginal troubleshooting.

The truck has 1992-1998 standard heads. Can I replace those with the 1999-up Performance Improved (PI) Heads, without changing pistions?

 

yes, and be prepared to use premium fuel…
be sure to use the copper anti-seize on the threads, when installing the spark plugs…
check the cam timing chain guides, as they are probably due for replacement…
new guides will save a larger repair job later..
make sure you are using the weight of motor oil ford recommends.
5-20, 5-30 simi- or full synthetic… 7500 miles between changes…
this will help the start up noise and wear these engines are known for. and any oil starvation issues….for parts like the chain guides…
and while your at it, new wide band o2 sensors and a cleaning of the air sensor in the intake inlet..