GM does not make the 350's in the trucks anymore but my 98 chevy 350 "R" motor does well. In a step van with around 10K lbs it gets between 12-14 mpg driving at full throttle. It's ok on power. Has a 14x6.5x6.5 box you can stand up in. Plus you can styand in the cab or streach your legs out on the dash.
Frank:
I don't know anything about the Ford spark plug problem but have a few tips. For AL cylinder heads never take the spark plugs out hot. The plugs are steel, expand at a different rate then the AL and do not come out well when warm causing dammage to threads. Sometimes the threads are dammaged but the plug will still go in and is blown out latter. Allways put an anti sieze compound on the spark plugs with AL heads to help them come out good next time. AL heads can be repaired with thread inserts sometimes without major work.
Tom
