they are a different design chamber. bigger valves and compression is slightly less when stock. but if u mill em down they will go up.but the gt40's are real good port them, go to a junk yard and get em off an explorer, they are like 80 dollars, than ur have ported for 5-600 and ur golden. ported stock heads are good but u need a lot to make them good.

 

You can port the stock heads to gt-40 specs for alot cheaper than buying gt-40s. The explorer heads are a good way to go but I'd port them just the same.

 

it would probably cost the same to get a set of stockers ported as it would to buy a set of gt40's you can get them off of ebay or on a message board for around $200, and if you actually find a pair in the junk yard they are only around $80

 

just bolt a set of 40's on and be done with it, you'll be happier than if you ported out some E7's. keep your eyes open for pre-worked 69-79 351w heads too, they are almost as good as the gt-40's in stock form but need hard ex seats installed. if you can find a pair on ebay thats already done for cheap that may be a route to go as well.

*edit* oh yeah and the differance between the heads are the gt-40's and early 351w's have larger intake ports and valves than the stock 302 heads, but the gt-40's flow a little better on the ex side than the 351w's do.