About the Holley carb... on my second 1968 PK21, ($100 ex 440 Maryland SP unmarked TYVM) had to do some Interstate driving to get it home and traffic was rush hour so cud'int go too crazy, but driving around town for the next 2 weeks got me cussing as I thought the damn secondary's weren't opening (vac). Wasn't till I got it somewhere that it could stretch it legs so to say... he he nothing wrong with the secondary system they just opened up at 120mph! Nice howl sound too.
Played around with some lighter springs but didn't really notice any difference.
Don't really understand the hate for the Holley carb, they were the carb of the day and after I swapped my 440 6bbl into the Maryland car and got sick of the 4 to 6 mpg & 12mpg max (gas was heading to 'gasp' 75¢ a gallon) I put the blue manifold & Holley on it, RESULTS!
On one recorded run at a average cruising speed of 95mph gas consumption was 15mpg with that little Holley. These 440 SP cars would have no problem 'Pining Out' the speedo.