documentation/references: 全 being pronunced `qian` in Southwestern Mandarin
I am working in Chengdu. But I don't think they don't really 前/全 不分, they can easily speak 全 though maybe sounds a little different on tone with 普通话. Maybe the pure ancient 四川话 has the difference as you talked. But nowadays, it's not so obvious. Maybe caused by the develop and immigrants from other place, the 四川话 also changed day after day. 所以，如果如今的四川话和过去的不太像了，你也不必感到奇怪。