Yes, 351 heads would fit but you'd have to get those reducers since 351 heads are made for a 1/2 inch bolt.
As to whether they are better than GT-40 heads...I doubt it stock, but if you get them ported well, they probably are. But I don't know for sure so take that with a grain of salt.
The best 351 heads are the 69 & 70 heads. They are probly not as good as GT40 heads, but can be ported to be much better, more material to be ported. In the mid 70's, the 351 & 302 heads were the same.