I assume you are using the HEI with a V-8. Most of the time there will be clearance problems with the firewall using the HEI. I ended up having to notch my firewall and I still had problems. The vac advance and wire connector section were about 180 deg apart and both were close to the firewall on each side. I solved that problem by rotating the cap 90 deg relative to the distributor body. That can be done by carefully measuring out and filing a new notch in the distributor body 90 deg (this is the notch that fixes the position of the cap. Now, the wire connector extension on the cap points forward and no more firewall clearance problems. When you are at the salvage yard, make sure you get the two-wire connector that connects to the distributor. Regular slide on connectors can be used but the special connector designed for that distributor works better.