'452 heads are the wild card with the larger intake guide boss. they did use a positive seal but i never considered it a good choice. i used to buy some positive seals for a 3/8" stem from Goodson (similar to the edelbrock seal) some years back. guide needs to be trimed to .625" to use them. to be honest i've never found any seal easier or more effective than an old umbrella in a stock or near stock build.