There are no ambiguities to resolve. You just ignore spaces when reading source code, except in the special case of a hollerith constant.
Your last example GOTOP(123,456) is not an arithmetic statement. An 'expression' is not any kind of statement. (Chapter 3).
It is also not a valid assigned goto. The compiler, I suppose, could attempt to guess what particular mistake you'd made, but it is still an erroneous construction.
FWIW, I used to write FORTRAN IV programs on 80-column cards that only permitted use of even-numbered columns. Though we did have a supply of cards pre-punched with 'C' in column 1.
According to this specification, FORTRAN II first identified an arithmetic statement by seeing (1) an equal sign not within parentheses, and (2) not followed by a comma not within parentheses [else it would be a DO statement].
If not an arithmetic statement, the 'beginning' of the statement is looked up in a dictionary in order to classify it.
The FORTRAN II code posted in a comment by @texdr.aft shows that the compiler accumulates a 'token' one non-blank character at a time, until it can find a match in its dictionary. It thus recognizes 'GOTO' as a keyword, prior to further analysis.
Per that, and assuming FORTRAN IV was broadly similar, your error case would be 'an incorrect assigned goto' rather than 'an incorrect arithmetic statement'. But it hardly seems to matter what sort of wrong it was, given it was wrong.
