limpwell- I would carefully measure the height of the straight 6 and try and estimate where the front of the valvecover will sit under the hood. You may not have enough vertical room between the crossmember and hood with the additional length of the 6. The carb will need to sit pretty level and not tipped forward or back. The 4's and V-6's are shorter and don't interfere with the downward slope of the hood.
