Jacob, yeah, these guys are right. SM420's are plentiful if you want to keep stock. Jump on Hotrod's offer.
An overdrive tranny will give you a significant decrease in rpm's however (provided the final drive ratio is around .72 or better - lots had .86 which is not very good).
Your final ratio is 1:1 with your 4 spd. Use this rpm calculator to compare between the 4spd and a O.D. unit.
rpm calculator
