Just finished rebuilding the engine on my '91 5.0.No machine work,just re-ringed the pistons,honed the cylinders,new rod & main bearings.The lower end is bone stock with stock cam.I added a set of Edelbrock performer heads(used),performer intake,65mm throttle body,73mm C&L mass,24# injectors,FRPP roller rockers,and 1 5/8" headers.The car runs great,but when started has a puff of smoke.After that no smoke and runs great.I suspect valve seals because I bought the heads used and like an idiot installed them without taking them apart and checking them.I pulled the plugs and found that #4 & #8 plugs had signs of oil on them.My question is before I spend an afternoon replacing valve seals,is there anything that would cause the rear plug on each side to get oil fouled or is this just a coincidence?