First, gear ratios. I have a HD 3 spd and I imagine yours might be the std 3spd
unless someone swapped a remote shift HD out of an F3. My 1st gear is significantly
different than yours at 3.71 vs. 2.78. 2nd gear is close to the same. But I also run
215/85R/16's so that evens things out some. I can start out in second gear if I
ease into it, so it's my guess you could probably get away with 3.08 in the rear.
With 3.50 in the back I cruise at close to 65 at 2500 rpm. Personally I think
anything between 3.00 and 3.50 will accomplish what you want.
The swap: The 9" rears from '57 thru '72 F100's will bolt in with only a few mods.
You'll need new U-bolts to hold the axle to the springs. This will be
upside down from the stock orientation. U-bolts can be had at NAPA or your favorite
parts house. You'll also need to fab a new bracket for the u-bolt on the bottom
side of the springs. I reworked a pair from a parts pickup by drilling new holes
in a 2.5x3.5" pattern. (springs are 2" and the axle housing is 3" + the thickness
of the u-bolt shank) I think theu-bolts I got were 3"x9" - 3x8 would probably work.
Shock brackets - I just cut the old ones off the 9" and relocated them
the same distance from the backing plate that the stock axle had them or find a pair
of shock brackets that someone has parted off an F2 (which also has a 3" axle tube)
This would solve the shock and the u-bolt issues at the same time.
Driveshaft: The 9" assemble is shorter than the stock Dana by about 1 1/4".
My 48 F1 has a sliding yoke on the front of the shaft. I simply went to a local
driveline machine shop and bought one off the shelf that was 1 1/2" longer. I've
heard some driveshafts are different so you'll need to improvise here. One solution
I heard about long ago was to get the differential yoke from mid-70's stationwagons
that used a longer yoke??? Or have your driveshaft lengthened.
Pinion angle: This needs to match (opposite) the output shaft of the
transmission. The flatheads I've seen are at about a 6 deg down angle so your pinion
wants to be at 6 deg up angle. Practically, this isn't too critical as long as you
get it close. I found that 5 deg caster wedges from NAPA got me within about 1 deg.
Put them between the spring perch and the spring pack. NAPA calls them 'shims'
so if you're in need of a pn, it's 704-1168
That's about it. Oh, as a bonus, the backing plates off the Dana/Spicer
41/44 bolted right on the axle stubs of my '65 9" axle. That was nice 'cause I
had just rebuilt my brake system. Have fun!
ps.
9" axles from '73-'86ish are about 3" wider than your stock axle you can make
this up with backspacing on your wheels if you want to go to the bother.
Also 9" axles from '67 to '72 use wider brake shoe for more stopping power.
Also your flathead will love 28 spline axles - no need to look for 31's
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