There is not a SFI 25.3 chassis spec. and there is not a SFI chassis spec for certifying down to 8.50. The following is from the SFI website
http://www.sfifoundation.com/
Spec # Full Bodied Cars Price (US$)
25.1E Full Bodied Car Tube Chassis – 7.49 Seconds & Quicker; 2,800 lbs. Maximum 35.00
25.2 Full Bodied Car Tube Chassis – 7.49 Seconds & Quicker; 3,200 lbs. Maximum 35.00
25.4 Full Bodied Car Tube Chassis – 7.50 to 8.49 Seconds; 3,600 lbs. Maximum 35.00
25.5 Full Bodied Car Stock or Modified/OEM Floorpan and Firewall with OEM Frame or Uni-Body Construction – 7.50 to 8.49 Seconds; 3,600 lbs. Maximum 35.00
Spec 25.5 does not need the diagonal braces in the floors if it still has the stock floor pans, where as the others will need them. To be certified down to 8.50, it needs to have the cage per the NHRA rulebook (welded to 6"x6" plates or to the frame, lower outside rocker bars if the floor has been modified more than 2'x2' for transmission removal or no outside frame rails, dash bar if the firewall has been modified, 1 5/8" x 0.118" mild steel or 1 5/8" x 0.083 chrome moly, and bars connecting to the subframe connectors or crossmember if welded to plates on floor. The lists do go on, but to answer your question, I have seen the difference between a 7.50 cert and a 8.50 cert being one bar (not having a kidney bar) or it can be so many more as not having a funny car cage, roof diagonal, floor supports, foot bars, floor diagonals, X-braces, and etc.
Bill Mitchell