So a few years back I bought some 18" rims at AMA Vintage days. I think they came from the Cannondale Motorcycle factory after it closed. So I got the bright idea to build a new rear wheel for my RT3 using that rim. I didn't realize until last night its a 2.15 width and the RT3 has a 1.85 width rim. Just out curiosity, has anyone laced up a wider rim on their RT or DT? Tire charts say a 100/100 X18 will still fit, but not sure if I'll have enough clearance around the chain.

I've used a wider 2.15 wheel on a monoshock DT175 without issue...  only about 1/8" wider on each side. Not sure about RT3.

Bigger concern I'd have is the drill pattern and angles of the rim holes. 

    I never thought of that. I just assumed that all 18" 36 hole rims would be the same. How would I be able to tell?

I don't know of any way except trying to assemble and see if spokes get tweaked because hole isn't aimed correctly....  

Maybe one of the wheel experts have a better answer.

Not an expert but probably built many at work over 50 years. The difference in angle of nipple hole is obvious when you get a hub with a big brake drum & small on the other side. So have to look & think about it. If you have the correct rim for the hub & fit it the right way the nipples will be relaxed & in a straight line. But get it wrong & they'll be pulling at an angle & won't last like that. I'm from back in the days of getting a handful of blank spoke then bending the ends to short & long bends for inner & outer. Then laying the rim over the hub on the bench supported at the correct offset. Then slide a spoke or two in & cutting off with a little out the top. Then get the old Cut Thread Machine & thread up 36 spoked--or was it 40 in those days of BSA wheels. Build them up & clean up the overhanging thread so as not to puncture the tube. Later we went upmarket with rolled thread dies but still cranking by hand. Don't get me going about re-corking clutch plates. Thumbs are still suffering lol.

Not sure if this video is helpful or relevant to your situation & his hub is the same size both sides so can't go wrong in his case. Just the first one i found googling. Plus link to where i found it.  How to Lace a Motorcycle Wheel | BikeBrewers.com
How to Lace a Motorcycle Wheel - YouTube