1/8th or 1/4 mile? I run the same size tire w/ 4.10s and a converter that has about 7-8% slippage... it's spinning 6800+ through the traps at 125+ MPH in the 1/4.

Given the mid-range that a tunnel ram could have, you may not be as far off as you think w/ your current converter.

EDIT: Assuming 4.56s don't cause the engine to over-rev and actually lose MPH on the big end, they'd add another 400+ RPM on top of what 4.10s would turn.