"Is there some rule of thumb for this?"
Yes - if the red light comes on and the engine seizes it's
too small.
Seriously - if you're talking racing you need to leave
plenty of air space above the oil. I race a BDD which has
basically the same oil system to your Twin Cam and we
usually put in 8 or 9 quarts [it usually blows out 1/2 qt
in the first session, that's how find the right level, so
say 8-1/2 qts]. So on that basis you probably need a
slightly larger tank.
I've no experience of street driving but I suspect you
could get by with less oil and a smaller tank.
Call David Bean Engineering - they'll know.
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