If you live near an agricultural area, drive around the back roads a bit. Look for 10 wheeler ag trucks sitting out in fields, behind cellars, etc. The truck can be any make, I have driven GMC, International, and Ford trucks hauling potatoes and grain that had 4 speed brownies. Or, ifyou can find a wrecking yard that has a few of these older big trucks, or better yet specializes in them you are in luck. Looks like you are in California. If you can find a place like that, grand. If not, try getting ahold of Holst truck wrecking in Ucon, Idaho.
http://holsttruck.com/ They have 40 acres packed full of trucks of all types. If I still lived there, I could help you out with another yard that has been closed for 15 years, they had alot of trucks of the right vintage, but now you have to drive up and ask to poke thru the place.
As a side note, I will risk sticking my foot in it since no one else has mentioned it. If you do not know how to shift without using the clutch, you will need to learn. The clutch does nothing for the brownie, it is not a syncronized transmission. I have been toying wth the thought of sticking one in my 68, but never have. Upshifting a brownie is pretty easy, downshifting going up a hill and under a load takes practice, patience, and the willingness to screw it up a few times. They will do what you want to do, it jut takes time to aquire the skills to drive one. I think one would be great in these trucks with the NP435, since there is such a titanic jump between 2nd and 3rd gear. Just my 2 lincolns worth.
Offio
When in doubt, hit the gas. It may not be the right answer, but at least it ends the suspense!!
The only substitute for Cubic Inches, is Cubic Money!
1967 F-100 swb "Varmit"
1968 F-250 4dr lwb "Beast"
1976 F-350 drw 460
1977 Club Wagon
1986 F-250 lwb