The 1953 RLDS edition introduced the infinitive form believe instead of the correct past-tense believed, as if the conjoined predicate was of the form “but would believe those records”. There is actually evidence for this kind of conjoining of predicates in the text:
Here the conjoined predicate is missing the would and is equivalent to “but would suffer that he might depart in peace”. Otherwise, conjoined predicates include the entire verb phrase and are in the past tense:
The critical text will, of course, maintain the earlier reading in Alma 3:11 where the conjoined predicate is complete and in the past tense (“but believed those records”). The 1953 change is probably unintended since the resulting phraseology is rather awkward. The source for the error appears to have been the preceding infinitive form believe in “whosoever would not believe”.
Summary: Maintain in Alma 3:11 the past-tense form believed in “but believed those records”, which is conjoined with the preceding “would not believe in the tradition of the Lamanites”.