73 340 with 051 heads

Rumble nailed it. Their all such similar castings there's no use in buying different heads unless you run across a screaming deal on some that you can trust. One of the biggest problems with any older heads is most of the time their so worn out when you get them that it'll cost nearly as much to have them rebuilt as it would to buy new heads that perform much better right out of the box such as the RHS X or EQ magnum heads.