There's 3 4spds, smblk, B, RB... 340/23 spline, 383/23 spline, 440/426 18 spline, there's also 2 different input shaft bearing flange diameters, so depending on the bellhousing being used, transmission being used, there's a lot of room for problems to arise when buying/sourcing parts, all the E body transmissions use the tailshaft shifter boss, the transmission external dimensions are all the same regarding bolt patterns/trans mount/ overall length/etc... many combos are possible with aftermarket bells, or machining factory bells..... I have a HEMI 18 spline 4spd behind my 340/6 with a factory bell and an 11 clutch, technically not doable, but with custom machining made possible..

Mike