Lower gears and still slugish.

I just finished my 8.8 swap. Going from stock 2.76 to 3.73 gears. I have a 71 dart with 318 and a 904. Have several upgrades on motor thought lower gears would make up for lower torque 318. Think the tranny is stock, I serviced it when it was out of the car. New oil an filter. There was some material in the pan but nothing crazy. I can here it shift all gears but its a slug. Can't spin the tires, feels like I'm always taking off in 2nd. Also messed several times with kickdown linkage, still wont work. You guys think tranny might just be wore out? Converter problem? Doesn't rev up like its slipping. This car should be pretty fun to drive with these gears but in barely goes.