There are a few catches with converting to the L26 UIM. You'll loose the PVC system since it differs between the manifolds, and I'm not sure if the IAC valve will work. I've also read about the L36 egr stovetop not fitting in the L26 UIM. You'll also have to drill and tap some of the openings in the L26 UIM to make it work.
In terms of performance a L36 throttle body(no matter what kind of adapter you use) will not flow nice with the L26 UIM opening. Heres why:
A Series 2 L67 throttlebody would work better since it has a round outlet close to the size of the L26. But some tuning may have to be done.
If your main concern is UIM failure, just drill and tap the coolant holes in the LIM and don't worry about it.