Is the kickdown linkage set up right?

You can modify a two barrel throttle bracket to work, but if you have the money (less than $40), this will make it easier:

MOPAR 1968-73 340 Small Block Throttle Cable Bracket 4-Barrel Dart Duster Cuda | eBay

You'll still need to lengthen the kickdown rod, but that's not too difficult. See above.