I am a big fan of the Dave Brown round back engine mounts:

http://www3.towerhobbies.com/cgi-bin...?&I=LXB872&P=7


Tend to use these on most everything. Notice the uneven hole pattern, this is to allow access along the sides of most two stroke glow engines from the front with a long tool. This is extremely helpful when the airplane has fixed cowl cheeks or tubs with any mounting orientation. The round back will allow fitting in many tight cowls where the humungous square adjustable types are just hopeless.

Now the technique I prefer is to mark the center line on the firewall as above wherever you want the propshaft to come out and mount the engine on the mount.

Next hold the fuselage between your knees sitting down and look straight at the front of the airplane and rotate/position the engine/mount exactly where you want it.

Use the Great Planes hole locater:

http://www3.towerhobbies.com/cgi-bin...&I=LXK378&P=RF

and insert it through the mount in each of the four firewall holes and turn a few turns. Remove the engine/mount and you have four perfectly located firewall holes to drill and install T nuts.

John