converter

I have a 91 with a GM Crate HT383 motor and a 700R4 (or the 4l60 without the e). The trans guy put a 3500 stall converter and its sluggish. My thought is it should be a 2800 stall but looking for advice to get the most power (Highway and Offroad) Gearing: 4:56 Tires: 37.5" Attached power...