I have a 77 F150 4x4 with a 351m with c6 and was wondering what everyone thought about whether I should rebuild the 351M or replace it with a 300 6cyl that I have in another 79 4x4. Both trucks are automatics and I am planning on taking the 205 Tcase out of the 79 and putting it in my 77 which is a 203. Any thoughts or info on milage you are getting would be appreciated.

 

If you rebuild the 351M, might as well make a 400 out of it. All that is required is a crank and pistons for a 400. I have heard that the rod are identicle. They are a fairly stout motor and with some aftermarket parts, they can produce some respectable numbers. Build for torque.

The 300L6, IMO, is the BEST motor Ford (or anyone) built. Durable, reliable, easy to work on, ect. They are not high reving, HP producing engines. They do have good low end torque. Also they can be built to make some pretty good torque motors,however, they are "only" 300 cu. in. Again build for torque.

Either engine will get ~ the same mileage. Remember, you have a 5000 lb. brick with the aerodynamics of a garage door. Mileage is based on several things. Engine efficiency, gearing, driving habits, terrain, ect.

IMHO, I wouldn't want a C6 behind a 300. I have a NP435 behind mine and it moves OK, but I think a C6 would take too much of what the 300 lacks-HP.

Check out the "335 Series" forum for specs/suggestions/opinions on the 351M/400
There is also an "Inline Six" forum for the 300.

 

You would need to replace the entire engine/transmission assy since the 351M and 300 bellhousing bolt patterns are different.

 

I have a parts truck with the 300 6 cyl and the c6 to take all the needed parts from. Was just curious what everyone thought of the 300 6cyl. I have only owned one prior to this and it was a fuel injected 300 that we swapped in to a 80 f150. We switched it back to a carb and it performed well. I was looking into a 460 swap but the cost of oil pan, engine mounts, headers would break my budget. I am currently running the 351M on a dual fuel propane setup and it is pretty doggy. I think the low end torque of the 300 would work well as I am using the truck for a hunting/wood hauling truck.
Thanks for all the feedback on my post.

 

The 300 is a great motor but I'm not sure it would be worth the swap. Personally, I would rebuilt the 351M.

 

I'm putting a 300 in my ford which presently has a 400. A lot of people seem to say they get the same mileage but for all the posts I've seen on mileage in both the 400 and 300 forums as well as people I know with these engines, the 300 consistently performs better. Going from 9mpg to 15 may not seem like much but it's almost twice the mileage. Also you should read the forums on the 300 here and on the inline 6 website.. you can build them up pretty cheaply.

That said of course, you're going to have 75% the cubes you would if you went to a 400.. it really depends on your priorities. Personally I'm not a power monger so it doesn't bother me. In fact I prefer really torquey motors.. I'm used to driving diesels though. I feel bad revving an engine over 3 grand .

On second thought, I think you should really keep the 400 and give the 300 to me