This is a very difficult situation, I understand your anxiety in choosing to do the operation when you've had a negative reaction to anesthetic. I've been through it, but not with one as old as your fellow. But, I urge you to deal with the situation sooner rather than later. Talk to your doctor and ask to be referred to a Veterinary College
or a specialist or work out a dosage protocol so that the dog is "just" under and not out-to-lunch under.
When my girl was under, it was a hard bad road out. I had to lay down in her recovery cage with her and help her come out of the drugs. It broke my heart, but, she needed the tumor taken off. It too wasn't "YET" malignant, but, in time....who knows.
At any rate, try to find out if there is any way to lessen the drugs negative effects... And, it may help to remember, that our Mastiffs have rather short lives. 10 to 12 years at the most. So, the balancing act is how much pain, discomfort or "weeping and growing" the two of you can tolerate. Would his life be better if the growth was gone? If the answer is, "yes", I'd move ahead.
Let us know,
E.