They've got the 350 confused with a 250. I started rebuilding them in the mid-1970's, taught the system for years, and have seen bunches of the 350's run without a kickdown cable with no problems. Since the cable bellcrank doesn't even touch the internal linkage until full throttle is reached, I feel the need to call BS on your transmission builder's claims. The vacuum modulator controls the shift points, not the kickdown cable. I can provide the hydraulic routing diagrams to prove it if necessary.
Jerry



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