Modifying headers, what size collector should I use???

What headers were on the motor when it was dynoed?If it was the tri-y then you have a monster.If it was dynoed with a 4 into 1 long tube header then you will drop quite a bit of power with the tri-y.

I would consider selling the tri y and putting the money towards a better header (for your application).

Those power numbers could easily run a 2" primary 3.5 collector on a lighter/lower geared car but will still pound the asphalt with a 1 7/8 primary and a 3" (3.5" would be better).

Even a 1 3/4 primary and 3" collector would beat the pants off of the tri y headers at your power level.

If you modify the ones you have a 3" collector would be my minimum.