Not sure if it is the largest, but PFORT would be a good one: https://doi.org/10.1002/spe.4380040405. I’m not sure if it has any extensions. I have a feeling, the PFORT authors may have used it to test their own program.
A mirror can be found here: pfort: Portable Fortran Verifier
Also J. Burkardt has a copy (might be modified): PFORT - FORTRAN66 Verifier
The M3RK ODE integrator (https://dl.acm.org/doi/10.1145/355887.355897) has been verified by PFORT, and is hence F66.
ALTRAN would also be a good one (https://dl.acm.org/doi/10.1145/800204.806280). But I don’t know if the sources are available anywhere.
Estimation Subroutine Library (ESL), see A naive question about single precision - #5 by ivanpribec, would also be F66, probably with some extensions.