904 misbehavin'

The car is a '64 Dart GT, but was originally a slant six car. The P.O. told me that they jacked up the '64 and drove a '74 under it. So, it got disk brakes, 8 1/4" rear, 904 trans, 318 w/Thermoquad and it definitely got the spring and slotted rod. Would that spring being weak cause the problem I reported? Would it prevent the kickdown rod from returning, as the first response suggested? Thanks!!
P.S. 10-4 on your sig, 65!!