What size rims?

This summer I have certainly joined the ranks of Norton owners the world over. Take a nice long ride, rectify a problem. Take a ride, rectify a problem. Mostly simple things, but nagging just the same.
I have two questions. First, my rims are in poor condition and need replacing. I intend on having new alloy rims with stainless steel spokes built this winter. Does anyone recommend a particular brand of rim, and what size is best for the front. I originally planned on going with 18" F and R, but a gentleman I called for an estimate said I should stick with a 19" on the front. Opinions?
Next question. My cylinder head gasket is leaking and I'd like to replace it this winter also. How difficult of a job is this with the engine in the frame? I realize that this question is relative, but I still appreciate any advice. While the head is off I'm considering having it gone through. Can anyone recommend a good shop here in the U.S? Thank you for any help you can offer.
Chuck

rim sizes

See Norbsa's string about Tire Sizes. The URL has an article that says stick with the 19" front and back for most Commando riding. I go along with this, but will probably lace up and extra rear hub with an 18" WM3 rim just to see how it works.

Getting the head off a Commando while the engine is in the frame is a trick operation. The pushrods have to be pushed way up into the head to provide clearance then the head is rotated backward then out sideways. It's pretty difficult even when you've done it before, but is possible. Read Commando Service Notes from the NOC.