st300, th350, and 4 spd m/t shaft's are the same length, use the same yokes, and should work.

There are 3 different th400 tail shaft lengths, and the trans yoke is bigger.
Short for F bodies.
Medium for A bodies(only seen a few).
Long for B bodies.

Unless you can find a good ready to go 350 shaft, i would do what Greg suggested.
Most places will make a new shaft for about $125-$175.
At least thats what it cost me.