In a car, I prefer to use a .12; the smaller engines will give you more power in the upper rpm range where you run most of the time. I’m sure others may have different opinions.
Try looking at the .12 OS CV-R. This engine is a true runner that has driven many a car to the checkered flag. Easy to tune and help is always available.
The .12 Fantom FR12 is a screaming meanie, it tops out at 43,000 rpm and will kick some a ss. It’s a bit more finicky to tune but once you get it figured out you will love it.
Have you thought about a Picco, Nova Rossi or Mugen Seiki .12 at all?
I just ordered my Mugen Seiki MT .12 bighead. I'm getting TONS of positive feedback about this mill. Runs cool, holds a tune ROCK SOLID and puts up the RPM's as well.
The 2.5 is a rear exhaust.
Do you want to convert your header and pipe to accommodate that?
And yes, the 2.5 will be on backorder for many months to come.