Is he going to stick with the 273 or put in a bigger SB?
The reason I'm asking is the RPM drop between each gear on the wide ratio trannys may not help that little motor on a road course
You should convert the tail shaft and output shaft to the later type that you can use a conventional slip yoke and U joint on that end of the driveshaft
The E body tranny have a completely different longer tail shaft and out put shaft designed for the E body shifter mount
The 1970 AAR Cuda and T/A Challenger and all the 1971 and later OEM 833NP trans. had a closer ratio gear set in them which may be better for road racing