What are the significant differences between the J-car and Mk IV chassis?
Ford J-1 and J-2 where built with the breadwagon lobster nose. Ken Miles died testing the J-Cars at Riverside and the second chassis was destroyed for testing J-3 was the first J-Car with MK-IV body for test mule. J-4 won Sebring in 1967. Ford took four MK-IV J-cars to Le Mans with J-5 winning the race (J-5 thru J-8 Le Mans cars). J Car name comes from Appendix J of FIA rule book that allowed Ford to race them. After Ford won Le Mans in 1967 the rules changed that prototypes could run only with 3 Liters engine (making the 427 (7L) obsolete) and sports cars (Ford GT MK-I) with 5 liters. Ford won 68 & 69 with MK-I GT-40 with Gurney Weslake 5L engine. After Ford quit J-9 and J-10 (our car) were sold for Can-Am racing in 69 and 70 with many different Ford big block engines. Ford built 12 tubs (2 destroyed). The spare 2 tubs were finish many years and were not marked by KarKraft (J-11 and J-12). The car sold by Gooding early this year was J-12 (white).
J-Cars were designed by KarKraft (Boss 302 TransAm, Boss 429, Ford GT-40 MK-II and IV) The MK-IV was 100% US built and the tubs were honeycomb aluminum built by Brunswick Aircraft Corporation. Ford won Le Mans with a 100% US team with AJ and Gurney driving J-5 to victory.
To answer your question Ford reinforced the chassis after ken miles accident and added a NASCAR Cage for security. That save Mario at Le Mans when his car was destroyed. J-3 and J-6 are in private hands and J-4, J5, J7 and J8 are in museums