Doing a 4-speed this winter...

Someone will always come around and say that they have a 23-spline behind a 700 hp Hemi, and race it with slicks all day long. Sure. The truth is a 23-spline will not hold up for long in a race car with any kind of power and sticky tires. I would NEVER trash a good 23-spline purposely... Drag racing with a 4-speed is spendy and requires alot of parts. Not just one good transmission. I've got 4 and a roll away full of gears including extra 18 spline cluster gears that cost as much as an entire 23-spline tranny, each.
The 18 spline gears are beefier, they are bushed, they have a coarser pitch, and they are further heat treated. 18-splines are the only way to go unless you move into an aftermarket 4-speed. That would be my recommendation.
"67 up pedals will work, but the assemblies are different after '71. Same clutch pedal, but the brake pedal is different.