So i rolled a 1968 ford last winter and have the pick up just sitting here and here within the next month i plan on yanking out the 390 and c6 2wd tranny . I'm in the process in trying to buy a 1978 ford f250 4x4 and will have it here within the next month and i was wondering bout the tranny fitting in because i already know the 390 is not a drop in motor for that year .Will i be able to keep the c6 tranny on it and just add a transfercase ? And I'm wondering if that tranny will even fit in that year ?

 

ok i had the same issue... band new 390 and a bum 351m in my 78 f150 ranger custom; it is a straight cab short box... it has a 3 inch body lift in it so it made it a bit easier, but to get everything to line up i had to move the motor mount holes up an inch and a half, i did not have to change the position of the mount towers or the hight, after you move those holes its pretty much a bolt in from there.
and with the tranny, as long as the 78 you but is a c6 as well, all you need to do is switch the tailshafts.

 

I have a '79 F-530 4x4 with a 400/NP435/NP205 combination. In the late '90's my ex-wife could not drive the stick, so I swapped in a C6 transmission a 2wd F-150 I bought. Along with the C6, I purchased a new 4x4 output shaft, clutch pack pressure plates, valve body springs,& non lockup torque converter for a 1990 F-350 4x4 w/460 engine, along with a wrecking yard t-case adapter. I swapped the output shafts while rebuilding the transmission with the clutch pack pressure plates & valve body springs I also ran new 1/2" (-8AN) cooler lines with a remote spin-on hydraulic filter plumbed through a B&M Super Cooler first the through the radiator (1976 F-350 w/ 460 & C6 w/AC) cooler to warm it back up to the desired 185 degrees then back to the transmission. Additionally, I replaced the t-case mount insulator for the t-case to frame mount on the driver side frame rail. Every thing worked well for a few weeks, but the twisting with the NP205 attached broke the transmission case along the top bellhousing bolt webs. The 351/400 cases do not have the reinforcement webs on the top of the case that the 429/460cases have, so I bought a big block transmission from a wrecking yard and swapped the cases. Soon after, I broke the aluminum adapter between the transmission & t-case. I had used the 60's & 70's style transmission mount that uses a triangular shaped body with a single through bolt at the bottom where it bolts to the lower plate assembly, so I changed the mount & plate to the 80's & 90's unit that uses a flat bottom with 2 studs to bolt it to a flat topped plate between the mount & crossmember bolted to the bottom of another adapter. It all worked well for about 6 months before the adapter broke again. At that point, I had a steel adapter fabricated (the factory aluminum ones are super thin in spots), only to break the top of the bell housing area again. at this point, I just returned the pickup back to the 4spd. It should be noted that the only pre 1980 pickups to use a transmission mounted t-case were 1/2 tons with the NP 203 full time 4x4 units. In 1980, Ford was installing the much lighter NP 208 t-cases. I am not sure if a 208 or BW t-case will work in place of the 205 unit or not. The newer units are much longer between input & front output centers than the 205 units.