The quickest and easiest thing that you haven't mentioned is adjusting the linkages. You want to be certain that pushing the go pedal down to just above the kickdown switch result in the ip giving maximum fueling. This frequently can cause a problem. Also need a clean air filter.

Assuming you've already knocked off those, does the tranny shift at the maximum shift points on the speedometer? If the tranny down shifts too fast you'll have a hard time achieving proper 0-60 speeds.

After that, I think you probably are looking at something more involved, like IP timing.

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Tjohn

82 300 SD
77 450 SL (gone)