g77 needs to provide, as the default source-line model, a “pure visual” mode, where the interpretation of a source program in this mode can be accurately determined by a user looking at a traditionally displayed rendition of the program (assuming the user knows whether the program is fixed or free form).
The design should assume the user cannot tell tabs from spaces and cannot see trailing spaces on lines, but has canonical tab stops and, for fixed-form source, has the ability to always know exactly where column 72 is (since the Fortran standard itself requires this for fixed-form source).
This would change the default treatment of fixed-form source to not treat lines with tabs as if they were infinitely long—instead, they would end at column 72 just as if the tabs were replaced by spaces in the canonical way.
As part of this, provide common alternate models (Digital, f2c, and so on) via command-line options. This includes allowing arbitrarily long lines for free-form source as well as fixed-form source and providing various limits and diagnostics as appropriate.
Also, g77 should offer, perhaps even default to, warnings when characters beyond the last valid column are anything other than spaces. This would mean code with “sequence numbers” in columns 73 through 80 would be rejected, and there's a lot of that kind of code around, but one of the most frequent bugs encountered by new users is accidentally writing fixed-form source code into and beyond column 73. So, maybe the users of old code would be able to more easily handle having to specify, say, a -Wno-col73to80 option.