Best to think of it this way:
F5M31/KM210 fibrous syncros = easier engagenment, shorter life, 89 CSM/90 DSM style internals = oddball setup, GL4 only
F5M33/KM215 metal syncros = seem to last longer at higher RPMs, GL4 only, higher torque capacity, shares more internally and can be modified in many more ways.
Either rebuild one or the other, don't play with both. Gearing is nice of autocross in the KM210, but I suggest getting syncros that will last longer, more so with heel and toe in the metal contact variety - meaning the F5M33 FWD DSM transmissions. Components seem to be easier to source for FWD DSM transmissions, but there are setup much the same way in both transmissions.
5th gear is not the same between both and that is why you have to swap transmissions (to the DSM FWD) to get the EVO3 fifth to fiit and get you lower highway revs.
I just about put my head in a vice and gave it a whirl after nearly a year or more of agonizing over what I should do about transmissions.
I got my car 4g61t car with ailing syncros in the original KM210.
Installed another working but well used KM210 from West Virginia. Paid $125 US plus probably $150plus duty/brokerage.
Those syncros last a few years, but I would get lockout at higher RPMS. Shifter assembly worn out, and syncros.
Decided to go with the 91-92.5 DSM FWD transmission due to spline count and the ability to install a Quaife ATB LSD (from the AWD platform W5M33).
25 spline and with 2 sets of shims, speedo gear and ball and cage bearing, you just need to see a competent trans builder to make it all happen like it's supposed to.
There are many things to finesse when building a transmission and it's not something that I have wrapped my head around, nor do I care to.
Get it built correctly and pay someone, if this is not a forte. Trust me, there is LOTS that can go wrong and you will be putting money into this venture either way.
You will be swapping the shifter arms from the KM210 to the F5M33, and the roll pin pushes out towards the firewall. I used an air hammer of substantial potency and a modified bit.
Good luck drilling whatever it is made out of.
Also, I believe you swap the shifter cable brackets that mount to the transmission also.
My thread with pics is here:
pics of scatter shield start here and
This is the bulk of it
Install a 4G shifter assembly to a 3G base, with all new bushings. Mix and match the parts, it works out. Double nut the main bar, be generous with grease in between everything.
Brass/brass-oil or plastic, just get new ones for every pivot point. The thread details it all.
Work out a clutch and pressure plate combo for your driving style, and please realize that ultra light flywheels will cause all kinds of noise, input spline wear, gear chatter/rumble and idle nuisances. Aim for the 11+ lb territory, or help a light weight unit with an SFI silicone filled Fluidamper or ATI multi=piece. ATI requires removing the timing belt to install it because it incorporates the gear into the setup. Pricey on both counts.
I'm sure there is more I am not thinking of, or forgot to touch upon, so I'll leave it to the others to chime in.
I'm tired and need some sleep.