My 1966 D 100 currently has a 318 poly with a 4 speed with a granny low and a 391 rear end. The speed limits on the freeways around where I live are 75 and most people run 80. With the current set up I have, it is hard to get 70 out of the old girl. I am sure someone has done the switch I am talking about, so I was hoping to get off easy and have someone tell me what I need to pull this off. I am told, Yes you can. The early 518 non electric is good or the electric that you can wire a switch for the od.The poly block only has one bell housing bolt that won't line up. Most just leave it without.No big deal.You will have to make some cross member mods. I only installed one in a car that required floor mods, I assume that a truck will be easier. Being it's a truck the 518 is the same as a non od 727. So guys, this is the first issue I am seeing. The 518 trans is the trans I was discussing with my transmission guy. The problem is that he claims the 518 would only work with the 6 bolt flywheel arrangement of the 318 engine, I have the 8 bolt version. Also, the 518 option requires driver input to switch on and off the overdrive feature. Failure to turn it off when not needed could potentially burn-up the transmission, according to my guy. Any comments on these issues? I am open to any and all information and guidance!