Those 781's flow very well, especially with the larger valves. The company I work for builds a 496 with those heads, a flat tappet cam and they make 480 hp and near 600 ft lbs of torque.
I just swapped a set of 781's on my '70 Caprice's 454. I lost a full point of compression over the oval port closed chamber heads, 10.25 to 9.25, but actually gained some power. Mine still have the stock 2.072 valves in them too.
1965 Impala SS, 383, 4 spd
1970 Caprice LS-4 454, TH400