F40, you don't mention the cost to recondition the existing head and replace the head gasket. I would expect that course of action to be much less expensive than going with a new engine. Having said that, it would depend on the history of the existing engine. If you have owned the vehicle since new, or with reasonably low mileage, and have used Mobil 1 with regular oil changes I would opt for reconditioning the head and installing a new head gasket. The replacement engines are a crap shoot, unless you or your mechanic know their specific maintenance history.

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Fred Hoelzle