Was thumbing through Leo's book and on page 78 saw that it is apparently possible to use larger diameter lifters.
Chevy Diam= .842"
Ford Diam= .875"
American Motors= .904"
Says: "The larger Diameters are used when the rules specify flat tappet, but prohibit mushroom lifters".
So you must be able to bore the lifter bores and use bigger lifters.......
