Hello!
Welcome to the board, In 1970 the S code 390 engine is not available.
Only : H code 351-2, M code 351-4V or Q code for 428-4V no shaker, R code for 428-4V with shaker.
See this links for more details :
http://1970mach1.homestead.com/enginesspec.htmlRaynald Bélanger Webmaster
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MCA Certified Judge 1967-70 Mustang & Boss