I have a chance to buy a '78 F-250 with a bad 400, with a T-18, for 100 bucks - decent truck, really.
I have an '86 carbed 460, complete, for an automatic.
Will any modifications be required to get the '86 460 with a externally balanced flywheel and clutch to bolt up to the 400 bellhousing and T-18??
(I know I will have to use '70s engine mounts - I have a spare set.)

Thanks in advance!

 

Your balance problem will be fixed when you buy the new clutch & flywheel. The headaches are in the motor mounts, accessories wiring & mount locations, and perhaps the pilot bushing. Given almost ten years difference in age, there are bound to be a few changes you'll have to make. The good news is that, overall, it's a relatively easy swap to do -especially if you have a donor vehicle for parts. Now is the time to put in a new (straight up) timing chain and ditch that emission compliant carburetor. If it is a high-mileage engine I'd put in some new valve stem seals too.
You will love your 460 powered '78 when done.

 

Thanks for the input.
I put in 460/C6 combo into my '66 F-100 to replace the 352 Fe/ 3sp OD - and you are absolutely right, I love the 460 with the correct cam timing.
Can you be more specific about the changes you reference?
My concern is the depth of the 400 bellhousing vs the 460, clutch depth, transmission input shaft length, crankshaft pilot hole, and the like.
I am HOPING it will be a bolt up solution to the transmission.
Can you confirm it will be bolt up solution?
Peripheral issues such as radiator, accelerator linkage etc I can handle easily enough - been there done that on my Slick.

Here is what my Slick looks like now:
‘86 460 bored 60 over with stock '86 heads - no porting
'77 F-250 460 engine perches and mounts
'69 straight up cam gears, Cloyes double roller with RV type cam
Edelbrock Performer (not RPM) intake and 1470 750 CFM carb on a 1" phenolic spacer
'76 Duraspark II ignition
Stock aftermarket manifolds with a 2 1/2 inch dual exhaust (plain mufflers); copper gaskets
'93 serpentine belt brackets and pulleys
'78 power steering pump, hoses with ’93 PS pulley
'76 power steering box '76 power steering pitman arm
’92 power steering cooler, mounted in the frame
'70 steering column
'66 radiator (re-cored)
'66 alternator with '93 serpentine pulley and fan
'86 water pump
'86 fan clutch with '77 7-blade fan
'83 C-6 from an '83 F-350 fifth wheeler, with '77 C-6 linkage (internal shift rod and kickdown)
2.75 rear end

'83 stock rear sump oil pan, with stock oil pump, pickup tube and dipstick (includes main bearing bolt with stud to hold pickup in place)
Includes '93 A/C compressor, but A/C is not installed. (Needed for the serpentine belts.)

’76 F-150 3-inch rear brakes

The only modifications I had to make were:
Fabricate an accelerator cable bracket by combining an FE one with a 385 one.
Shorten the front driveshaft to adapt to the C-6 (with a conversion joint)

Change the C-6 internal shift linkage from a ’68 – ’79 C-6.

 

Just wanted to put in my two cents. The 351M, 400, 429, and 460 all have the same bellhousing bolt pattern. All I read says that bellhousings from all these motors are interchangeable. Your truck is going to be a BEAST!! T18 behind a 460? Wow. Granny low gearing plus big block power. I am considering a similar project, NP435 behind a 460 in a 1992 F-150.

 

If you use a flywheel for the 86 and a diaphram style pressure plate it will be a bolt together deal.

 

You most likely will have to do some grinding in the bell housing to accomadate the added depth of the fly wheel clutch assy. also you may have to cut around an inch off of the collar that the throw out bearing slides on. do not force any thing. These are the areas that I had trouble with when installing a 429 in my 79 f150 with a 4 speed.

 

the external balance flywheel is thick, grinding on the bell housing webs is mandatory for pressure plate clearance. use the inner smaller pilot bearing hole in the crank. use a 400 pilot bushing or one from a 300 six. i cant remember which one it is.