I hope the path results show the lump is benign. Regardless, here is my 2 cents worth on the surgery:
Rickie had a lump on his leg last year that turned out to be hemangiopericytoma (malignant but not invasive). My regular vet removed it. The tissue was sent to the lab, wasn't stained, got mucked up in transit, the path lab couldn't tell if the margins were clean.
We got the answer this summer when it returned. This time I had a surgeon with a superb reputation at the local specialty clinic do the surgery. It was a major excision and required a skin flap from the stomach, but he got clean margins (just).
My lesson learned was, don't mess around - for anything but the routine, get a specialist you trust to do what is needed.
I love my regular clinic and vets, but the difference is that the specialist does this every day, all day, and he's very, very good.
The other advantage of the specialty clinic is that they have 24h care.
I don't post often but will be watching this thread and hoping for good news for Monty.