Why do you want eight carbs ? Good luck setting the mixture.
If you " must " do this, how about a blower manifold ? Aluminum
strips could be machined to fit the manifold, the strips could be
bored and drilled for carbs. As for carb, Motorcycle carbs might
work well, as for linkage, probably the eccentric bellcranks from 2
4 cyl motorcycles would work as well as anything. Using 2 throttle
cables might be better than a Y splice.
Am I right that your out for torque [ tractor pull ? ] if your
out for top end, dual quads, or a big Demon type carb would rule,
as 8 wimpy carbs won't flow as much air as one monster carb.