I have the pre-rail, slotted push-rod hole, '65 289 heads. At how high of a camshaft lift, do I need to start worrying about the length of the slot in my rocker arms? I have a new cam with a .471 lift, intake and exhaust. I did not want the extra expense of roller rockers at this time. If the stock rocker arms do not have enough slot, to make the most use of the cams lift, can they be modified (ie: lengthen the slots)? Any suggestions of a less expensive alternative to roller rockers?