I've been away for a bit, so this a little late. I agree with the stock convertor replies.
A stock 354 has pretty good bottom end torque, start upping the cam and it goes away (moves up in the RPM range) pretty quickly. With the overdrive I'm assuming you going to be running a 3.50 or deeper gear and that with the 2.45 low gear in the 518 should make it a pretty decent combination, especially in a fairly light truck where traction will be laughable anyway.
1957 Plymouth (Hemi, Dual Quads, A833 4 Speed 9 1/4 w 4.10)
1937 Dodge Pickup (Hemi, 6X2 intake, 46RH, Dana 60 w 4.56)