just wondering if it is possible to shim them differently so they will hold up being run in reverse which is how counter-rotating alphas are done. anybody had any luck?

Problem is the units shifts in the lower with a cluth dog. The front gear has a tapered bearing, and the reverse gear has a flat roller bearing that will not hold up to a full load.

Just buy a sei counter rotating lower.

You can use the same housing and swap out the internals, but it will cost more in parts than just getting a complete counter rotating gearcase. Basically you replace everything but the vertical shaft.

I am not positive but I think you can buy a 3.0 left hand outboard lower unit and mix match parts to make it work. I believe the main part are the back gear, bearing, the prop shaft and prop shaft carrier. Check parts lookup and see if the parts interchange. Left Handed lowers should be very inexpensive on eBay.