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Discussion Starter #1
I just got a new 4-speed tranny cause i broke my other in half! The New one is off a 390. I've been told that I need to lengthen my drive shaft. Anyone know if this is true.


70 Mustang Fastback modified 351W Auto to 4-speed conversion
 
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Discussion Starter #2
As far as I know there are two types of top loaders when it comes to length, short ones and long ones. Short ones come in mustangs , so if this tranny came out of a mustang you should be OK. Also , since this is out of a 390 car you tailshaft housing will be wrong for the Hurst shifter used on the '70. Take the tailshaft housing off of your other tranny and use it on the 390 trans. While you are at it you should replace the seal in the back of the tailshaft housing and also check your speedo drive gear. I am assuming you already had a 70 4-speed toploader in the car, looking further at your post you mention that this is a 4-speed conversion. So I'm not sure if you already had a '70 4-speed and '70 shifter assm. , if you didn't just ignore that tailshaft housing business. Also if you still have the old toploader they are very easy to rebuild , you might want to consider this because the trans you just got may also need work. It's not uncommon to have to replace syncros and second , third gears as well. These trannys are 30 yeaars old and have usually had their share of wear and tear.

Good luck

70 SCJ R-CODE MACH 1
88 GT
 
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Discussion Starter #3
I did have a 4-speed in it already, but it was a 65 tranny out of a mustang wih a 289. I would have loved to rebuild it, but the tail shaft was twisted so bad i couldn't even get he yolk off with a sledge hammer. As for the rest of it, the casing was cracked from one side clear to the other. (literally broken in half) The tailshaft didn't look in good enough condition to be used again. The new tranny out of a 390 came from a torino or galaxie (I don't really rember) The shifter position is in the middle of the housing. I am using a hurst competition plus shifter. The new one is 28 spline and I think the old one was 25 although I was told by the person who sold it to me that it was converted to 28 (yea right! ) but I don't really believe him now that it is to late.
 
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Hmmm... This complicates things. Sounds like you have a "LONG" version of the toploader. I would think there would only be a few problem areas getting this thing into your car.

1) Will the speedo cable reach ?? not sure if the placement is the same on a long tranny.
2) You will need a new yoke ( guess you do anyway :) )
3) Will the shifter mount in the correct place ?
4) Custom driveshaft ?

Perhaps someone on this board has already been through this swap ?? Another ( more costly ) alternative is to change the main shaft in the trans to a short shaft and get the proper tailshaft housing. When I rebuilt my trans I spent a fair amount of time talking with a fellow named David Key, he is a toploader Guru and all around nice guy, he might now the easiest solution to your problem. Try this url for David Kee http://www.4speedtoploaders.com/

70 SCJ R-CODE MACH 1
88 GT
 
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Also, I believe the input shafts for a bigblock and small block are different. I have been told NOT to use a big block toploader on a smallblock due to this. I also think the shafts are thicker(bigger)??

Full Throttle
http://homepages.go.com/~ful_throttle/fullthrottle.gif
 
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Discussion Starter #6
Ah yes ... That is another flavor of toploader, small spline and big spline. 390's use the small spline version that is shared with all other small blocks. So even though the 390 is considered a Big Block it uses the small block style toploader. Only 427,428 use the big spline , I think 429 also , but this is not my area of knowledge , and there are probably others too ( trucks ?? ).

70 SCJ R-CODE MACH 1
88 GT
 
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Discussion Starter #7
Thanks. I confirmed this with a local transmission mechanic. I found the parts finaly and I should have the tranny ready for instalation in about 2 weeks.

1970 Mustang Fastback modified 351W auto to 4-speed conv.
 
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