Years ago Mopar racing manuals used to say if you switch a lightweight (2800 pounds) BIG block car from a 727 to a glide it should be a wash. The added torque of the big block could handle the gear ratio reduction. In a small block even the swap from a 904 to a 727 is about .10 with the right convertor. More with the wrong convertor. I know in my Duster at 2850 pounds and a pump gas 408 combo I am taking a performance hit but for a bracket car to me the loss is worth it in consistency and a lower rebuild costs. My cores cost me 50 dollars with a 1.76 gear and rebuild kits are under 150 dollars.