i stepped out in the garage and got some seals out and did some measuring. if the head has an un-trimmed guide boss then there won't be a big difference between the chevy umbrella and the mopar due to the chamfer at the top of the boss. the mopar umbrella is molded to fit the chamfer. when the boss is trimmed a little; removing part or all the chamfer, the chevy umbrella will give more coverage because it is molded flat on the inside. I prefer the most coverage I can get. there's a few things to look at here and as aftermarket parts are used instead of factory more thought needs to be given to coverage. more exposure of the stem/guide will lead to more oil in the chambers when using umbrellas and I've actually had umbrellas work better than some of the so-called positive seals.