The rocker setup on my race car had the end springs eliminated and the
locating springs between two rockers were altered with a number of coils
removed and then the spring was stretched to control the spacing. Does
this help? I am not sure but I am guessing that they may have honed out the
rocker bushing slightly larger than stock.
I did notice that these rockers move considerably easier than on a stock
setup and couldn't figure out why other than the spring setup.
The bores in the block for the lifters were honed .001" over. (they
were using the shortened lifters)
Another interesting detail which I could never figure out was the use
of the early double bladed chain tensioner. They didn't use the later
design with the piece of rubber. Did they know something that we don't when
they built the motor in 1967? I don't know but a guess would be it had less
drag. At least somebody building the motor thought it was a better design
for a tensioner.