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The 215 does have side mount locations, but that was only used in cars. On the 215, the front mount for trucks was on the front cover under the crank pulley, and the rear mounts are on the bellhousing ( all 215/223 6 cylinders and Y-block V-8's mounted this way in trucks ). The 240/300 block has side mounts only. I think it might be possible to mount the 300 in your truck by using aftermarket small block Ford mounts, but you would basically need to change the engine and transmission both ( my brother swapped a 302 into a '67 pickup with a 240, and all he did for the front mounts was swap the towers side for side to get the mounting location farther back, so the mounts should work the other way around ). Since the 215/223 bellhousing doesn't fit anything else, you will lose your rear mounting position, so, so much for keeping your bellhousing and transmission if that's what you had in mind. I think a 300 w/C4 or AOD would be a cool swap. I,ve seen one F100 with one in the flesh and a picture of another in a magazine.
A nicely built 300 with a C4 is exactly what I am hoping to accomplish. Only thing that scares me is the fact that the 300ci is a bit longer than a 215. I've also never tried fabricating anything like an engine mount. Oh well, guess there is a first time for everything.
I think you should be able to use the aftermarket crossmember mount for the small block Ford ( someone also makes a weld/bolt in side mount kit as well ). The tranny mount crossmember everyone sells works for all the automatics. Lengthwise it might be a tight fit, but I bet it will work.