I just traded work for a 91 Explorer with either a cracked head or blown head gasket allowing water into the crankcase. I have tried to find a good site for info but came up with nothing. I would rebuild it but was told the older 4.0's were junk and I should go with the newer style. There is a new one on ebay for close to $2000. I figure if I rebuild the original I will probably be close to $2000 with machine work and parts especially if the heads need replaced. Now the question-are the old ones junk? How much trouble would it be to convert it to the OHC 4.0? How much trouble to put in a 5.0? It is for my daughter so more economy would be better than horsepower. Any advice would be appreciated. Thank you.