Generally you want to temporarily mount the cowl before the engine and then measure the distance you have between the front of the cowl and the firewall. Mount the engine so the prop hub is 1/16" to 1/8" further from the firewall than the measurement you took. Depending on engine selection, a larger engine may have to stick out further if it's as close to the firewall as you can get it. This won't hurt anything, it just doesn't look as nice.