I hope all works out, I have red flags going in my head with some of the values you have been given is all. You can give whipple a call they should have a good idea for a 4.6L mod motor what blower rpm will be required to get you 28lbs so you aren't throwing money at multiple pulley combos.
Pulley combo was a 3.5 upper, 7.40 lower. Made about 34#. Believe this was on the street and off the gauge, not entirely accurate but nonetheless it wasnt where we want it.
Based on the info provided by Lethal. A 4" and 6.55 would land close to 28#. Every .125= 1# on the whipple 4" and the 6.55 is equal to -4#. Also account for big cams, ported heads and anything else. We wont know forsure until it's on the dyno but really no "red flags".
4"÷.125=32#
6.55"= -4#
32#+-4#=28#