Touring and SS have so few entries at Regionals I think they can go almost anywhere. ITR will be VERY small at first and should be combined with ITS. T1 and T2 can run with Big bore. So:

1. ITR-ITS-ITB gets you to around 35 cars if ITR shows 3ish.
2. ITA-ITC-T3 gets you to around 30.
3. You put SRF with Small bore, SPU and Legends (SB gets under 10 cars...SPU under 5) for around 32 cars.
4: Wings and things: Faster formula cars and sports racers.
5: Slow formula cars.
6: Big Bore (Including ITE, SPO, B Prep and D Prep)
7. SM
8. SSM-SSB-SSC

The key is G3. All these cars are very similar in that they have fiberglass everything. Similar mindsets.

What am I missing?
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Well, maybe you don't remember the really BIG stink when SRF used to run with production.

Like the comments here, SRF and Prod make their speed in whole different places. It's a 5 times worse mix than having IT and SM run together. (IMHO)