I think you are going to end up with the 4:30s with that short 103 circumference. Where do you want to be RPM wise? Looking at those sheets I would say low to mid 7000s at the least.

My car is similar to yours with less power and with a glide, 4.30s, 106-107 circ. slick and a 6200 stall 8 inch converter I was over 7000 when I was running 1/4.

I only run 1/8 mile now so I run 4.88s. Shift at 6900 and cross at 71-7200. If you were only running 1/8th the 56s would be perfect I bet.