I believe SPO is a regional thing and is a catch all category for everything that falls outside the normal rule sets because they were too...something or another. SCCA rules are painfully strict, so it wouldn't take much to step outside of them.

Any vintage clubs likely won't be strictly SCCA based, but could be. Around here Rocky Mtn Vintage Racing uses SCCA/FIA/IMSA classifications and the date of manufacture for the car to determine classifications. They also have their own subsets of rules for general competition and safety that have to be met as well. However, they also have pretty strict rules about contact, appearance, and conduct in addition to being period correct for classifications so if you run 1970 specs stuff, you can't necessarily use the latest electronic gizmo.

W2W = shorthand for Wheel to Wheel racing. ie, not rally, time trials, hill climb, autocross or track time.

SPO does not have a national title championship/points, ergo it is 'regional.' You probably want to look at the GCR. The SCCA has announced they will be revising the website (again). I find searching best using an outside search engine. As Tony mentioned, its SCCA's fairly recent attempt to keep drivers with cars no longer classified.

Northeast and Mid-atlantic Vintage racing clubs:
HRG = Historic Racing Group. http://historicracinggroup.com/
SVRA and HSR rules touched on here http://www.forabodiesonly.com/mopar/showpost.php?p=1970082938&postcount=25

I'm pretty sure you'll need an SCCA racing drivers license for any of these clubs. So add that into your time and dollar budgets.

looking at a SP-2 spec'd 944. has run and made the podium a few times. Has SCCA and NASA log books. Is this a good class?