Just my opinion, go for the stroker kit. With just the head swap, and nothing else, I don't think you'd feel much of a difference. With the combo you have I don't think your Cobra Jet heads are a bottle neck. The CJ heads are pretty decent, lots of people who've run plenty fast with them, even with stock valve sizes. However, I do think you would feel a significant difference in power & torque just adding the stroker kit and the added cubic inches. And again, with everything else being the same, I don't think retaining your CJ heads would be holding you back too much. Although in the future, with already having the stroker kit, you'd be better able to take advantage of the better heads.