I'm assuming that it's a running/driving car right now. I would build a 360(same size/all brackets fit) w magnum heads(looks stock) proper piston/head gasket selection to get .035-.040 quench with a CR in the 10's(gives you max efficiency/power) & 68-70 340 HP ex manifolds(you said no headers). mag neads have 318 sized ports so bear that in mind as you select an intake manifold(sb street dominators work well 60-75$). When it's ready to go drop it in & there's less down time & the customer is out the door(& happy) quicker.