I measured the two transmissions from the bellhousing edge to the flange. I did it several times and several different ways to ensure I came up with the same number. I think the flange type makes a slight difference in addition to the OD type. If I recall it was 11.5" in my case. There is a certain amount of fudge room. That center support bearing rests on those pins and can probably be considered 'close enough' within +-1/4 inch or so. The spline between the center support and the rear axle can easily suffer more mis-length than the center bearing supports can.

Too short and the center support bearing will get pulled against the mount, too long and it might fall off those pins.

I guess you already have the hard work of getting the OD to fit into the minimalist PV tunnel done?