My buddies Chevelle just had a rebuilt tranny put in. It has a shift kit and very firm shifts. It also is very harsh when you shift from park…etc…

Is this what is expected with a shift kit and higher line pressures? Or is something else wrong? I believe it’s a 400 tranny with “stock” converter. It idles in park at about 800rpm.

What type of "shift kit" did they install?

I have no idea. I'll ask the owner.

Be sure the modulator is connected to full manifold vacuum first of all.

Harsh reverse engagment is common on TH400's and on modified TH350's, but if properly setup you can alleviate most of it.

Was the trans rebuilt?
Did they leave out the cushion plates, especially in the direct clutch?
Did they check the reverse boost valve for wear?

Elevated line pressures, high idle speeds, and overkill shift kits will add to the affect.

I don't have much info. and don't want to tell the guy it's too harsh after spending the dough to have it fixed.

I'll check the modulator like you mentioned.