The main problem I see building a /6 is head flow the ported numbers I've seen are disappointing, a six cylinder engine has to flow 33% higher than an 8 to be able to make the same power, So the rule of thumb for an V8 is 2hp for a each CFM a six would be 1.5hp, and best number I've seen is a 180cfm which would put you about 270hp that's probably 10-11:1 with a 286-296ish cam.
