Where do 302 heads crack?

The cracking is typically the result of the induction hardening process used on the valve seats, and like RRR pointed out it usually shows up between the valve seats.

But the cracking problem on the 302/308 heads is a little overblown. Listening to folks on here you'd think at least every other set of heads has cracks and is junk. I run a set of 308's on the 340 in my Duster that I had rebuilt and ported, they were checked and had absolutely no issues. I even bought them on eBay based off of crappy pictures. I also have a set of 302's that were on an '86 318 I bought and briefly ran in my Duster. I actually pulled the heads last night because I sold the engine today. 85k miles and the heads look great other than the bad valve seals.

Yes, they are more prone to cracking than other heads out there. But finding a good set isn't a total crapshoot, there are more good ones than bad ones out there. I'm 2 for 2 so far.

Why the 302's though? It sounds to me like you should be looking for something with more performance. The 302's are good for a little bit of a compression boost because of the closed chambers, but they still have the same 1.78/1.50" valves as all the other 318 heads out there. The flow slightly better on the exhaust side than earlier 318 heads, but the intake side is pretty much the same as every other 318 head.

A set of Magnum heads would be a better choice IMO, larger valves, better flow, same closed chamber head. If you get them from a wrecking yard you can pull the rocker gear, which is 1.6:1. And if you're changing the cam you can just get oil through lifters with the new cam anyway. Just need oil through push rods and a new intake to complete the swap. And finding something with magnum heads at the yard is probably easier than finding something with 302's nowadays.