1964 plymouth valiant Slant six to 340

Just FYI, if you had a later 4 speed that used a conventional u-joint, a driveshaft out of a 108 inch wheelbase A body with 8 3/4 rear and either 4 speed or 3 speed manual will be exactly the right length. Also (I think) a 904 automatic. 727 automatic is longer and a driveshaft out of that won't work.

Depending on the trans of the donor car, you might have to change the front yoke of the driveshaft. The driveshaft in my 65 Barracuda came from my Mom's 71 Duster 340 3-speed manual, and I had to change the front yoke to fit the larger 4-speed output shaft. No other mods necessary.

Now I did this a pretty long time ago. Used A body driveshafts in usable condition are a lot harder to find these days. Plenty of places can make you a new one, though.

BTW, the 64 trans you plan to use has a 3.09 low gear. Later versions had a 2.66 low gear. Multiply the low gear ratio by the rear axle ratio to get your overall low gear ratio. I've run both trannies, and let me tell you, the 2.66 low gear and a cammed 340 purely sucks off the line with 3.23 gears. The 3.09 low gear works great with 3.23 rear gears. The difference really is amazing. Since you'll be running 3.91 rear gears, the 2.66 low gear would work fine, and the 3.09 low gear might even be too much.