difference between 3919840 and 3964291 heads

[hotz]

hey guys,
Can anyone tell me what the difference is between the 3919840 heads and the 3964291 heads.??.  I have heard that the 840's produced more power and are a more desirable number. Is that because the 291 was produced in late 69 and doesn't fit may 69 cars? I have found that the 840's are a bit more $$$. Will the 291 heads make the same power as the 840's? thanks for all of your help.

[rodent]

The heads are the same except 291 had 1.88 exhaust valve and a tapered plug seat and the 840 had 1.72 exhaust and a flat plug seat.