Drivetrain Which First Gear To Go With?

Did some work on the 1969 Mach 1 this Saturday in preparation for the Devereaux/Kaiser Car Show in Sarasota, Fl on April 28th, 2024 (highly recommend this show).

Verified TDC on the balancer....indeed the outer ring shifted from stock by about 10 degrees. Then marked the tick marks with white paint for the each 2degrees and orange paint for every 10 degrees...I have to paint the pointer with a different color as with my bad eyes (I have one eye that sees near, and the other that sees far and neither of can see the pointer well with the timing gun in the way) I can't really be sure...but I think I'm a little too advanced at 12+ degrees BTDC

Then changed out the starter that was giving me trouble when the car got hot...turns out it wasn't the starter but the attaching nut for the starter wire AT the starter....it wasn't there....but went ahead and replaced the starter....and I'm glad I did. When I removed the old starter I could see the teeth of the flywheel being chewed up !! I did some research based on the current starter's part number (3132) and found that it has a 3/8" offset. I THINK that is the correct starter if I was using my old 351 T3 bellhousing, but I'm actually using a T5 bellhousing.... I put in a 3124 starter that has 3/4" offset. Starts fine both cold and hot.....I reached out to Modern Driveline for confirmation of the 3/4" offset if using a T5 bellhousing on an older mustang.

Now for the T5 confirmation tasks.
1 - I do indeed have a yellow 18th tooth speedo gear for the speedo cable
2 - I do have a yellow speedo drive gear in the T5 which seems to be a 7 tooth drive gear
3 - (here's some interesting things) the T5 body casting sunburst has a 96 which I believe means it is from 1996. (I have conflicting information that it was either from 1990 or 1993...but that is hearsay as I don't have a tag on the transmission)
4 - the belling housing has part number of E9 which seems to be 1989.


Took some measurements of the current yoke in preparation for the new center section.
looks like I have a Long Narrow 1310 with 1 1/8" cap u-joints

Right now between the flange of the T5 casing to the flat edge of the slip yoke it is .925" (I think the tail shaft extends pass the flange that holds the seal, right?)

My current 9" differential has the Long Narrow 1310 pinion yoke.

The people that I am going to purchase a whole third member from(QP), currently doesn't have the Long Narrow 1310. The have may have a Short Narrow 1310, but definitely have a Short Wide 1330 pinion yoke.

If the T5's tailshaft extends pass the flange, would I have enough spline contact if I were to go to a Short Narrow 1310/1330 pinion yoke, as that would be about 1" shorter in driveshaft length....giving me about 1" more visibility of the slip yoke than it has currently.

I'm wondering if I am TOO close to bottoming out the slip yoke currently.

I might undo the rear u-joint this weekend to see how much farther towards the T5 does the slip yoke go and measure the tail shaft's protrusion, and measure the slip yoke's length.

Any insight would be most appreciated

For a custom DS you put the yolk all the way into the trans and measure the centerline of the u-joint at the yolk to the u-joint center at the rear end and subtract 3/4”. This is at normal ride height. Street car suspensions typically do not have any more movement by the yolk than that.

Having it too far out means you have less engagement of the yolk on the output shaft so that’s up to you.

When I did mine I subtracted 1" for net length..
As said above, do this when he car is down and settled on its suspension... I have race ramps so it helped alot.. You can see one on the front left wheel/tire in the attached pic..
1350 u joints on mine..

Had too short a driveshaft in my first car.. A bad measure... I was too poor ( 15 years old) to have it redone so I always worried about it falling out or twisting the transmission splines.... It never did...