Never seen any with primaries that large, and not sure how they could get a 2.5" primary to match up with the largest ports on a BBC? Are your ports on the heads large enough to be 2.25"-2.5" openings? If not then making the tubes much larger than the ports wont work, and they'll still restrict flow.
Largest I've seen available were 2.125" tubes and 3.5" collectors.