Alterations that can be done to the PG1 to make a hybrid/custom ratio. Gears 1 and 2 are part of the input shaft. You must keep these as a matched set. gears 3, 4 and 5 can be swapped individually by moving both input and output sprockets as matching pairs. The limitation to this is a standard output shaft uses a 19 spine count to a U spec output shaft which has 27 splines where the 5th gear fits. So you must match the 5th gear type to the output shaft. This does mean standard gearboxes tend to have shorter ratios and U spec boxes have longer ratios.
There are variations available for final drive. The 4.2 and 3.923 can be either standard or U spec. A 4.06 final drive is always standard spec. The 3.65 final drive is either U spec or X spec (a standard size at the bellhousing and U spec in the outer case)
When working out gear ratios, a bigger number means shorter gears.
Photo shows a standard spec gearbox
This is a U spec gearbox.