Originally Posted by karolh

The clunk on the fill line is to aid emptying the tank as without it the line would just float around inside the tank and make emptying it rather difficult. There is really no need to pad the tank with a layer of sponge but it certainly can't hurt anything, as one of the advantages of using a felt filtered clunk on the feed line is to eliminate the possibility of air bubbles going to the carb. Also the hole in the firewall to allow for adequate carb clearance is quite normal for rear carbed engines.