For less than a swap you can get 400hp out of the 400m. Food for thought. The 6.9 isn't a good replacement IMHO.

 

If you're wanting to stay mechanical you could always go with a 12v

 

I think he is more looking at the fact he already has a 6.9. I wouldn't say a 6.9 is a bad replacement for a 400m. You might be able to get more hp out of the 400 but the mpg's on a 400 are terrible. If money wasn't a issue I would say go with the 12v. You can easily make more power on them but the 6.9 is a good reliable engine and if you are not worried about making lots of hp it will be fine.

 

Thats true

 

I have a 78 Bronco and I'm planning on swapping out the 400 for a 4bt at some point. A 4bt weighs nearly the same as a fully dressed big block so no suspension upgrades are necessary, they have a similar overall length so no cutting of the firewall is required. I think Im going to box the front half of the frame (at least around the motor mounts). I had a 6.9 in an 84 f250 with the auto and it got 18mpg. I've seen 400 trucks that get 15 with a 4 speed and 3.50 gears. I would say, in having experience with both 400's and 6.9's Its not worth the time and money. Either build the 400 for fuel economy with a 4 speed (435 is the best bet) behind it, or save up and swap a 4bt in there. A properly geared 4bt/5speed combo will get close to 30 mpg. At least thats my plan!

 

My 400 has twin 750 hollys sittin on top of her, I think I nearly drain half the tank on start up! haha