From the Monoposto Register web site:
The primary changes there are:
Inclusion of the Hoosier Formula Ford tires based on the lap time and wear experience in 2006.
Adoption of the SCCA 2015 GCR Kent and Cortina engine rules to replace ours. I believe the most significant changes are a reduction of minimum flywheel weight and the inclusion of aluminum heads.
Minimum car weights do NOT change.
Elimination of the requirement that HFF cars have steel shock bodies. Functional requirements are unchanged.