Transmission experts - 3 on the tree differences?

[TxFireman1972]

Before I start:

NO I don’t want to go automatic
NO I don’t want to go 4-spd or 5-Spd
YES I want to maintain my 3-on-the-tree

Now that we have that out of the way….

1968 F100
390FE
3-spd manual on the column
9” with 3.25 rear gears

I love the 3-speed and want to keep it but I would like to have overdrive. So thru my research I’ve ended up with 2 possibilities:

1) BW T85-R11 3-speed OD transmission. I do like this option but they are getting tough to find, especially in good shape with all of the needed parts.

2) GearVendors OD unit added to the transmission I have. I do like this option and would probably go this route if I can find a way to make it happen. Yes it’s pricey, but simple usage and would be all brand new.

When I spoke to GearVendors he told me they have a model that will fit 3-speed and 4-speed Ford manual transmissions as long as they have the longer 14” tail housing. So I put the truck on the lift and get under there to measure it and of course I have the shorter tail housing that it only about 8” which they don’t have anything for.

So I’m curious if all f100 three speed manual transmissions had the shorter tail shaft or is there a certain year or model 3-speed on the column that would be a direct bolt in but have the longer 14” tail housing? Thanks in advance for any help!