On a naturally aspirated engine, the difference in power is not that much, provided the '69 351W heads are fully ported with the bigger valves.
My brother and I have almost identicle 306 cubic inch engines equipped with dual quads. He built both motors. In an effort to avoid the amount of hours that he would have to spend on a pair of 351W heads for my combo, I bought a pair of 58 CC AFR 185's. On the dyno, his motor made 311 rwhp and 287 rwt, while mine made 307 rwhp and 278 rwt. As you can see, they're very close. Of course, if I were to add Nitrous Oxide, I would guess that I'd see more of a gain than he would. The aftermarket heads will allow more room to grow your combination.