My guess is fuel delivery issue...

[Barneyszoo]

If you can find them, there is another choice for the primary filter, used on the 1969 m656 8x8 series trucks, shown below. No filter element to replace, just turn the handle to clean and drain...

We use that same type of filter on the ink lines at work only bigger, they work good.

 

[gimpyrobb]

What needed drilling out?

 

[Loco_Hosa]

What needed drilling out?

The primary fuel filter has a slightly larger bolt running through the housing than the secondary's do. When I barrowed the little washer/perch/fitting thing from a secondary fuel filter assembly had to drill it out a little bit so it could fit over the bolt.

Other than that it fit very well and seems to be working great