This is an syntactically unexceptionable Conditional sentence of the type sometimes called the first conditional, which is used
to express a hypothetical condition that is potentially true, but not yet verified. The conditional clause is in the present or past tense and refers to a state or event in the past. The result can be in the past, present, or future. —Wikipedia
FumbleFingers' identification of a simple past in the conditional clause (protasis) is entirely correct. If the Mayans were ... cannot possibly be subjunctive, because the result clause (apodosis) is cast in the indicative. This is clearer if we substitute a third-person singular subject in the protasis:
✱If Tortugero Monument 6 were wrong, (then) we'll use &c is not English.
The meaning must be
If Tortugero Monument 6 was wrong, (then) we'll use &c
As for recasting to end into the “past infinitive” construction, which Borstal urges and FumbleFingers appears tentatively ready to accept:
This construction might better be called the perfect infinitive, since it is the infinitive of a perfect construction. Its function is to define its “Event” time as prior to some other “Reference” time — which may or may not the same as “Speech” time.
In the case at hand, R-time is not S-time but the past time at which the Maya were wrong. Casting E-time —the time when the Maya made their prediction—into the perfect would imply that they did not become wrong until some time after the prediction. This is absurd: the wrongness inhered in the prediction itself, the Maya became wrong simultaneously with making the prediction and continued to be wrong as long as they maintained the prediction. E-time and R-time are identical, and it is proper to cast E-time into the “present infinitive”, which collocates E-time and R-time:
You were wrong to say X yesterday.
You are wrong to say X today.
You will still be wrong to say X tomorrow.
EDIT: (in addition to adding the important qualifier syntactically to my first sentence):
FumbleFingers points out that the straightforward syntax of this construction is in this instance profoundly confused by the semantic implications of the content, and I think he is right. The discourse itself is about time—more, it is about a discourse about time. And verifying the hypothetical would in effect render meaningless not only the hypothetical itself but also very system of tensual (is that a word?) reference in which it is expressed. Dealing with those semantics blurs any sense of stability in the syntax.