I wouldn't knowingly tell someone they were buying something they weren't, but at the same time I could really care less if I was buying the truck and got a 360 or a 390. There isn't enough of a functional difference to matter to me or anyone else. If you have a 360 and are ashamed for some reason just tell everyone it's a 390. Nobody will ever know or care otherwise.
I would just be appreciative it wasn't a 351M or a smallblock.
The same thing goes for 429's and 460's. Since getting into the 385 motors the last few months for my Ranchito I've run across countless 429's advertised as 460's. Not that it's a bad thing since anybody with a few rocks rolling around up there will take an early 429 over a late 460, but the point is that these engines are not rare and most are uneducated as to how to tell the difference and when it all boils down it really doesn't matter.
Nor would I mislead someone. I would tell them how good the 360 was for me.
Except for a project where I built a Boss 460 I would take a 429 over the 460 any day f the 429 was as stated above an early 429.
As for a 360 to 390 both stock in a truck had the same size 2v and doing the same work to each engine produces both of them into better torque motors with the 360's internal design helping to produce a little more torque per cid than the 390.(part of a ft. lb. per cubic inch) It really depended where in the US that the 390 or 360 was more popular in installation. Mountainous Colorado and Alaska got many more 390s than 360s, Texas where I worked for my first Ford Dealer got more 360s in plane Jane models and 390s in fanciest models. Texas had almost no 240s or 300s come through our shop but Colorado had many 240s and 300s. So it depends where the trucks were destined for out of the Factory barring custom orders. Starting in 73 because of the gas crisis Ford released less 390s than 360s just for that reason. City Dealers saw more 302s and country dealers saw more FEs.
Old Fords Rule Was a Ford Service Tech
71 F250 with Shell Car 390 NP435 Dana 60 3.73s, PS, PB, 750 EC VS Holley Accel Points Eliminator.
98 Volvo S70 2.4T Auto 71 Service manuals Volumes 1,2,3 and 4 So ask away. My Gallery Spark test
ok all of that is great and all that. so back to the original question. no they arent very rare. why are you looking for one? i bet you can easily find one real close to you. if not if your truck doesnt have one you can build one real easily. crank rods and opistons is all you need to convert the 360 to a 390.