not that hard to swap final drive gears - Si transmission final drive is lower gearing than the ZC - the ZC transmission will bolt into any 88-91 CRX/Civic - the ZC uses a ZC intermediate shaft and equal length 90-93 Integra axles - no other stock 88-91 axles will fit - to use the Si axles in the ZC transmission, you need to swap in the Si differential - we do these ZC/Si hybrid transmissions all the time, using ZC 1 thru 4 gears, Si 5th and Si final drive - have done both kinds, with ZC differential and with Si differential - the steel shifter forks in the later D-series transmissions are not much better than the aluminum ones in the L3s - the aluminum ones break occasionally, the steel ones bend a lot - we see more problems from grinding gears with the bent steel shifter forks than we see from broken aluminum ones