A member just went through this, his handle on here is Dunnuk Racing, he recently did some cam articles for Mopar Muscle.
He built a towing 440 for his late 70's 1 ton reg cab for towing. I cant remember the exact details but wasn't he getting close to 20 mpg empty? Well over 400 hp and 500 lb-ft torque. I had it saved on my watched topics on the old forum but that all went bye bye on the switch over. Did a search but I can't find it.
I agree with most on here, get rid of the open chamber heads and build some quench in it. I'd stick with under 10:1 if you want 87 octane, if u step up to 91 you could probably get away with 10:1 ish. Also the small cam with as much lift as possible is also good (Hughes engines etc) I honestly think you could do better than 20 with FI and an OD