i'm credit-grinding in b-spec myself, here's what i'm finding works best for me...
car: red bull x2010 s. vettel (i earned it for hitting level 35 in b-spec), the formula gran turismo will work basically the same, just add about 25% more time for completion
race(s): dream car championship (i think it takes level 24 to unlock this, not certain)
i run the whole five-race championship.
(generally takes a bit over an hour, unless there's rain on either of the last two tracks. for the entire championship, i net 1,080,000 credits. [each race gets 108,000 credits plus the 540,000 credits for the championship.] for this, all i have to do is intervene and start each race.)
good luck!