I recently picked up a ford 9" to bolt into the car. I am also changing transmissions to a th350 with a 9" tail shaft. With what I am reading online, I can reuse the stock driveshaft with this trans and the stock rear end. I have also read that with the stock trans and the ford 9", you can use the stock driveshaft. So with that being said, I should be able to am I right? I know the only way to actually know is to bolt it all in the car and measure. However, the center section for the 9" is getting built and the trans is also about to get built. Also, I know that the stock driveshafts are the greatest things in the world, I will eventually upgrade it down the road.

 

Assuming that the axles are in the same place as stock (wheelbase is the same):

The 9" ring gear is 1½" bigger than the 7½". Right off, that means that the drive shaft will have to be half that much shorter; ¾"; all else being equal.

The 9" pinion gear is longer than the 7½"; not sure how much. I think it's in the neighborhood of 1¼ - 1½" longer but I'm no expert on that. That means the drive shaft will have to be that much shorter also, again, ceteris paribus.

Those #s of course, are with all else equal, including for example the transmission length and location.

You can set your new 350 next to the old 700 and see the difference there. Might be longer, might be shorter, iunno. Then you'd need to take into account the front <-> rear difference caused by the trans mount pad on the ext hsg. Might make a difference, might not.

Personally, I don't think I'd trust "reading online" to assure me that I could just jam the old part back in, and it would fit. Not saying it won't; only, my trust level doesn't quite reach that far, to "assume" that it will. You just have too many variables involved, too many opportunities for uncertainty and differences, too much "custom" stuff, to "assume" much of anything.

If I were doing what you are, I'd install the trans and rear COMPLETELY, get them set up PERFECTLY (pinion angle and drive line angle and such), then measure for the drive shaft I need. If the one I have fits, so be it. If not, time to get EXACTLY the one I REALLY want. Abuncha posting on the Internet isn't gonna help that very much.

 

Stock trans and a 9" you actually need a driveshaft that is about an inch longer. You can get away with the stock shaft, I've done it temporarily, but it won't have an ideal engagement. I can't offer any advice on the 350 vs. 700r4 length however, other then that I know people who have done the tailshaft swap and used the stock driveshaft. I haven't crawled under there cars to see what the engagement is.

 

How sure are you that the 9" is setup to take your stock DS U joint ? Having a driveshaft made to fit is your best bet and relatively cheap, I had PST make one for me for under $350 shipped when I put my 9" in

 

There are several different length yokes for the 9". I used a stock 3rdgen driveshaft for mine (with the 700r4) till the larger diameter driveshaft came in.
Not 100% sure on the th350, ive heard long tails will work with the stock shaft but I do not know.

 

Not at all actually. That's why I haven't even thought about buying the conversion u joint yet. I was just hoping to get some general direction since I can't figure out for myself right now.

 

I have a ford friend that rebuilt several 9"s, so I had him help me with this one. He said the yoke is about as long as one that you are going to get. I know a picture won't do much, but I took a picture comparing them just to see if the driveshaft would line up in the tunnel and all the mounts would line up. Also, the pos torque arm mount that is in the center section itself, was put on by the previous owner and is getting removed. I have a Currie one to weld onto it. However, you can see where the stock torque arm mounted onto both rears, and the yoke seems to be sticking out further with the 9" than the 10 bolt. But that is completely a guess at this point.

 

Here's a little more info

https://www.dennysdriveshaft.com/c58_ford_9_inch.html

 

When I put in my Quick Performance Ford 9", I still used the same stock driveshaft, when I had the TH350

 

9" tailshaft on the th350 or a short shaft?

 

The long tailshaft I believe.