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I own a 64 impala SS. Had a 327 with a 2 speed powerglide. I’m going to a big block 396. Was curious what transmission is better, the th400 or 700r4. I’m favoring the 700r4. I’m only going to cruise it on weekends, if that. Would like to know if anyone has installed one in a 64? How tough was it? Any insight would be appreciated.
 

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The 700R4 would be great with say 3.90 gears until you shread the internals when the 396 hits its stride unless its well beefed up. The 400 on the other hand will handle the torque no problem however they're not very efficient. That being said most everything will line up similar to the turbo 350 and 'glide without having to go with a custom made driveshaft, I'm speaking from experience with my old 68 coupe that was swapped from a 307 & 'glide to a get this... 455 olds & T350! High school what can I say? Everything lined up but I had to play with the linkage to get all the gears with the 350. Went like lightning in a cloud of rust.
The 700R4 is physically the same as the T350 short shaft and uses a throttle valve cable over the vacuum modulator, early generation was a 28 spline input shaft while the later 700R4 was upgraded to a 32 spline and stronger input shaft. Plenty of support for the 700 but supplies are limited. The 350 was the same as the powerglide dimensionally from what I remember, might have to move the rear mount around but the 700 will bolt to the back of the 396 and the converter should hit the inner circle of flexplate bolt holes. The 700 converter uses metric threads on the welded bosses.
 

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If you're looking for overdrive get the 4L80E. It's a TH400 with an extra gear.
It'll fit since it's longer than a TH400 but the same diameter.
The downside is it's all electronic. You'll need a TCM and a wiring harness.
 
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