Ported W2 heads, solid camshaft, different intake manifold for the W2's, and headers and you'll be easily in the 500hp area.... but you won't make 500lbs of torque without a stroker.
Our '72 Duster has 11:1, solid purpleshaft, strip dominator, 850, 1 7/8"s headers.... runs a consistant 11.00@122mph.... with some fiberglass.