Like Mike said, ET thresholds effect what safety requirements are applicable. Concerning seat belts, if you are running slower than 11.99 ETs, your stock or expired SFI belts are OK, but if quicker than that, yes, you need to replace the 5 point safety harnesses every 2 years, unless you are able to have the original manufacturer reweb them, and put on new SFI tags. Scattershields have the same ET cutoff, SFI 6-1 & 6-2 scattershields have a 5 year period of certification, but many can be sent back to be inspected and recertified, but if the bellhousing has been cut, welded on, or otherwise modified, they most likely will not re certify it. Lakewood stopped making new FE scattershields several years ago, not aware of anybody making SFI scattershields for FEs, other than QuickTime, unless you want to spend much more for a custom bellhousing from a company like Browell . Also, be aware that the SFI dates are issued at the time of manufacture, NOT at the time of sale, or any other time. So it is possible to buy a brand new scattershield or safety harness that has been sitting on a shelf, and is already expired, or at least is closer to the expiry date than your may be aware of. So such things should not be ordered until you are actually close to being ready to put them into service. Other SFI dated items can include the clutch and flywheel, helmet, and for automatics, transmission and flexplate shields. Always seemed silly to me, that a 12.00 car can use factory 50 year old seat belts, but at 11.99 ETs, new, 3" wide, 5 point harnesses are no good after 2 years, or how a 1/4" thick steel scattershield is no good after 5 years.But, I don`t make the rules, NHRA does.