A few more comments:  This D was set up for propane when they purchased and I noticed several of you talking about gas manifolds and/or exhausts.  Would that have made a difference if it could've had dual exhaust?  When they restored the tractor they kept the gas pistons which along with the dual exhaust makes for a stout D.  Don't really think this information would mean much to him as he has told me that old timers have told him more or less the same as what you have told me.  I was curious as I don't recall ever seeing a D with dual exhausts before this unit. Thanks for comments!