Re: problem with engine stumbling, then recovering upon
acceleration. Check the oil level in the carb dampers.
Take the air cleaner off and watch the pistons in the carbs
when you rev the engine. They should rise and fall in unison.
You mentioned that the carbs had been rebuilt, maybe the jets
need to be centralized. If they are a little bit off, the
pistons might be sticking. Check the oil first, if the oil is
gone, the pistons might be rising too quickly. I think this
would cause the symptoms you describe.
(Early GT6 with Early GT6 engine)