5.1 paths with 6.1 or later installed

I have an older design that needs to stay in version Quartus 5.1 / SOPC 5.1. I have installed some of the latest versions on my computer and I cannot seem to get the paths set correctly in my BASH window for compiling my embedded software. There is a batch file to switch from 6.1 to 7.0 and 7.1, but I have not seen anything for getting the paths correct for 5.1. Does anyone know what the magic incantation is to switch to SOPC 5.1 after 6.1 or later is installed?