Vacuum Spring Problem?

Guys, I seem to be having a problem getting the most out of the signet when I mat it around 60 MPH. For starters, I'm pretty sure my kickdown is adjusted correctly. When I mat it I get a burst, but for a very short time (maybe 3-4 sec) then it drops off like the secondries have closed. (my best guess) I did confirmed that the kickback rod is all the way back to the firewall and there is nothing stopping the secondries from getting fully open. I have changed out the springs before as I have the kit,however I can't seem to get it right. I do understand that I don't what them opening just crossing the steet or even fully open going 60. Due to it being vacuum sec, what is the best spring size to use to assist in holding the sec open "until" I let off on the gas?
Thanks.

I'm sure everything else matches pretty well.

273 Performer intake,
Carb: Holley 600 w Elc Choke
Stock cam