I did the same on my 81 Special II. What I did was go to this old shop called Wheel Works (not the chain store) in Hayward here in CA and got an NOS rim from the Chris the owner, I cant remember the manufacturer but it wasn't a Yamaha rim but it looks very similar. He got me a set of spokes and I took apart the old wheel and laced up the new rim. I then had him true and tighten the spokes for me when I was done. Take lots of reference pics if you go this route!! You could call him and get some advice if you wanted, he's really awesome, has tons of crazy stories and gives you tons of great info, The first time I called to ask him his advice, we talked for over an hour! Can't say enough about this guy... (510) 785-4396
but, yeah I think the 78 and 79 had an 18" but they had disc hubs. that was my problem too...