All the aftermarket (and factory) floor shift kits I've seen for three-speed tranmissions mount at the tailshaft, placing the shift lever significantly farther back than the SM420's lever is located. Even in passenger cars, it's difficult to install this type of shifter with a bench seat, since the lever (in unmodified shape) will be too close to the front edge of the seat.
I don't see how it would be any different in a C10 without making major modifications to the shift lever or using bucket seats.
Finally, if 2nd gear seems too high for starting off without a lot of clutch slip, why not use it like a true 4-speed? I've considered using a 3.08 or 2.73 gear in my 64. If I were to do that, I'd definitely use the "granny" 1st as the starting gear. It wouldn't be a whole lot different than using a TH700R4 automatic with its extra deep 1st gear.
