If you short shift car and ET stays same or improves, replace valve springs. Also, if it has a roller cam, pull cam and look at it, if springs are weak you will see where the lifter is landing on the backside flank.(jumping the lobe) In which case, thats why most people have lifter issues, the wheel splits after a while. As long as you claim the the springs have been in there, they are more than likely shot, unless you back off the rockers when car sits.
As for the fuel possibilities, if you keep throwing jet at the carb to satisfy top end plug reading and the bottom end numbers start to falter, its fuel volume. Good Luck