There are a few things to concider before you make large prints. At these enlargements the problems grow just as much. You will proberly have a problem with a noemal enlargement lens, since most are made for a 10x enlargement.
You will need something like a Rodagon-G lens otherwise you will get unsharp corners. A 80mm lens may help since they do cover a larger field, but they aren`t made for such large enlarments. Your corners will be well covered but the grain may be soft in general. Make sure your negativ, lens and groundboard/wall are all parallel. The most important is that the negativ and lens are exactly parallel, otherwise your corners will be unscharp.
Something like a lazeralignment tool will help there.
Stop down two stops not more, sometimes one stop is actually better.
Another thing is if the negativ gets too warm it will expand resulting in a grain which moves. It may help to warm up the enlarger for a few minutes.