The cars in GTL are from the FIA Historic GTC-TC league and so have to to obey to the FIA appendix K rules. One of the rules is the cars may not be tuned. I guess the 289's you're talking about do not obey to these rules.
3.3 Period specification
3.3.1 Period Specification is defined as the configuration of the model, proven to the satisfaction of the
FIA or the stewards, to have existed in the period in which it is classified.
3.3.2 Any additions to a car’s period specification are forbidden unless expressly authorised by the
regulations herein specific to the car’s Group, or by an announcement in the FIA Official Bulletin concerning
the specific car model or components, or imposed in Article 5 (safety).
3.3.3 Alternative components to the original manufacturer’s specifications can only be used if it is proven
that these components were used in that model of car in an event entered on the FIA International calendar
in the period.
3.3.4 The period, alternative parts and components, and the extensions of the homologation form to be
considered will be specified on the HTP applicable to this specific car.
3.3.5 Unless otherwise specifically authorised by these regulations, any part of a car must have identical
dimensions to the original part and evidence of this must be provided.
3.3.6 The technology used, including that allowed with homologation extensions, must be compatible
with that used in period.
For example, no rev. limiter can be electronic before Period H
