Only alternative? No.

You could buy an intake specifically intended for a pre-1987 sbc, so that the bolt holes would match.

You could have the manifold you already own welded and then redrilled for the proper bolt hole angle.

You could do as the web suggested (use a round file make the holes oval) and then use large washers to make up for the gaps. You could also use angled washers such as those sold by Summit, and/or by McMaster Carr (an inductrial supply company).