Just bought a 78 F250 4x4 which I am restoring.
The truck still has the standard straight 6 motor and 4 speed manual gearbox.
At the moment when it gets to about 60 mph the revs are pretty high.
I am considering fitting a Cleveland 351 in there , but what do I do to get the revs down a bit and raise the top end. I would like it to be able to do 80 mph without the revs running at nearly max. Do I need to fit another gearbox or do I need to modify diff ratios?

Thanks

 

Define "pretty high"?

You could do either or both but much depends what axle ratio you have now. I'd definitely suggest changing the trans to a newer Zf 5-speed to gain the OD ratio, that will even help the I6 cope better.

 

Well it is not running comfortable at 60 so much so that I do not feel the motor will cope with 80 mph for 100 mile trips.
Its hard to say exactly but it runs high revs at 60 mph.

 

I will start looking for a 5 speed transmission then. They are hard to find in South Africa though.

 

Do you know what axle gearing the truck has? I'd suggest changing the gearing but it can be an expensive job with a 4wd since you have to change both axles. Installing a tech would be a good idea even if it's just temporary so you know what actual rpms are.

 

This site has already helped me immensely with part numbers and other info. Just by checking the FAQ section. Thanks again.
I still have mullions of questions but I will cross those bridges as we get to them.

 

Don't forget to factor in the tire size you plan on using. I still like using: Gear Ratio Calculator to compare different configurations.

 

Thanks gfw very helpfull