If you have a cowl, and you have the plans, then you should be able to figure out where the cowl attaches to the fuselage on the plans. Place the cowl vertically on a flat surface and measure through the thrust washer hole to determine how long the cowl is. You should then be able to add this measurement to the front of the fuselage plans to get a total distance from the firewall to the front of the cowl. Add 1/16" or so for the thrust washer to stick out the front of the cowl. Then measure your engine (and mount) and see if you need to recess the firewall further than the plans show or build it further out to accommodate a short engine. Don't forget to allow for any right or down thrust required which will effectively change the amount of room you need for your engine by a smallish amount.