Slow going from first to second

YES trans shifting fine org linkage,and is forward when closed,if i leave it in D floor it it goes 123 just fine, if say at 25 to 30 in second gear i floor it, it will down shift to first and seams to hold onto first, dont want to blow up my new 360.thanks for hanging in on this.
My question was: was the transmission shifting correctly prior to installing the shift kit?