Matt's pretty much answered it.
The bits he didn't answer... the disks on a SM are the same as on your America. So the dementions you want are on your bike...
The Thunderbird Sport wheel has a 17" rim. You could run with that but the stock spoke wheel has a 18" rim. The t'bird sport wheel however is as rare as rocking horse poo. Some of the guys on here have them, and are just waiting the time/finance/enthusiasm to do the conversion.
As Matt says the upgrade kit is the best way to go, but is pricey.
wanna see some pics....
With a Triumph 18" rim on
As far as I know I am the first (on here) to actually get the conversion done with a T'bird Sport wheel. (and mert... IknowyouhavebutyouusedaSMwheelnotspokes) I haven't really ridden it far as the insurance policy only started yesterday, but to the mot and back...

I had a choice - rebuild after a crash with a 'normal' wheel or do the upgrade.

So while the bike was in bits...

If you wanna see it in person - come to
Rays' BBQ I'll be taking her there.