[KTL]

I don't think it's necessary to modify the housing to fit the later style allen head pin and big jam nut. Instead, simply shim the box/fork to remove the play. Find some washers that have an inside diameter that is the same size as the roll pin. Stack the washers to remove the play between the fork and the shift housing.

Just don't make it tight such that there is excessive resistance to rotation. You may have to leave a slight bit of play, simply due to the thickness of washers available to you. Nonetheless, a little bit of play is better than the large amount you currently have.