Ive raced sportsman class in AHRMA for the last 3 years, and at least in my region (mid atlantic) they arrange the sportsman class as follows:
they run a sportsman 125 set....
then the 250s and above are grouped as "Sportsman Open"
Come to think of it, its the same here for the GP classes, as some of my friends on YZ250s are racing YZ400s!
Its not just that they run the motos together...(I mean that its common for related classes to be out on the track at the same time) but the 250 sportsman machines are actually in the same class, and if your out there on an MX250, you will be fighting the CZ400 guys for a trophy!
I know that AHRMA lists sportsman 250 and sportsman 500 as separate classes, so in other regions maybe they run it that way, but I would check it out ( and then shoot for an MX360 maybe?)
Personally, I race a Kawasaki F9 350 in sportsman open, but i am building a 1975 DT400 with a full MX400 engine for next season. (I got a thing for stripping down enduros)
My Yammy will have to be "historic open" which is one notch later than sportsman which sucks because it will still have only 4 inches of rear travel but they lump it in with the YZ400 and MX400s which had mono shocks that year!
You will love AHRMA. Its such a great vibe. Theres competition a plenty, but its been my experience that the riders have a great respect for each other. Just walking the pits is a blast! Its like a vintage bike show...some of those guys are riding museum pieces!