BEST Spark plug - 383
The V-power. According to the chart in Mancini's flyer the R5670-5 is for 8.5~9.5 to 1 comp. and the R5670-6 is for 9.5~10.5 to 1 comp. Yours falls right in the middle. If it were mine I'd err on the side of safety and try the R5670-5 first and watch them closely to see if their hot enough. If not then I'd go with the R5670-6