The problem is that these are diffusely infiltrating tumors. They crawl right through all that subcutaneous fat in all sorts of funny zig-zags with little resistance from the normal adipocytes.
I would refer to an experienced sarcoma surgeon. Even the best surgeon could easily do another surgery and get another 2 mm margin. This is where an experienced surgeon counts. Is it really worth doing the surgery given the tumor, its location, its imaging, etc to possibly avoid RT when there's a good chance you're going to come back with residual microscopic tumor and close margins anyway on the re-excision and still need RT.
Without that, I'd radiate for sure. Don't skimp on the margins. RTOG 0630 is probably ok, but I'd treat with the larger fields. Would strongly consider boost to 66 Gy around the cavity for close margins.