Ineligible or Not?
 

Team A has a legal formation (5 lineman properly numbered and 7 man on the line of scrimmage.)

A74 lines up as a back, either as a tailback, fullback, or receiver. Is he ineligible by number and if so is there any exception?

For High School, A74 is always an ineligible receiver by number, even if he is lined up as a back. There is no exception and he cannot "report as eligible".

He's ineligible by number. The numbering exception applies on scrimmage kick formations, which allows teams to have less than 5 players numbered 50-79 positioned as linemen. FED Rule 7-2-5

Case book 7.2.5 situation D explains this.

Canadian Ruling
 

CANADIAN RULING:

Players are ineligible by position (one of 5 interior linemen) or by number, or both (in most cases). To legally catch a pass wearing an otherwise restricted number (40s, 50s, 60s in Canada), the player will have had to report to the Referee before the play began.