So I'm working on a project to increase the capacity on the factory alternator above 100 amps, and I came across this page on the Delco Remy website which I believe says the 27SI was superseded by the 28SI.
http://www.delcoremy.com/find-a-part/product-details/27si-alternator
The dimensions of the 28SI seems roughly the same as the 27SI, and they both seem to use the same "J180" mounting style. The 28SI seems to be used a lot on heavy duty trucks, school buses, etc. and is offered in factory amperages of 130-200 which slots in nicely above the 27SI.
I have an email inquiry in to Delco Remy to see if they can confirm this information, but wanted to check here to see if anyone may already be running a 28SI.
Looking online, it appears you can find aftermarket 200amp 28SIs for a little over 100 bucks... And Genuine Delco Remys for a little over 200 bucks.
Either way, I will post back here with what I find.