like mike said heat is your issue i personally dont like bg bore kits in the 300 and 400ex's due to them being oil cooled. just doesnt seem as affective to me.if your gonna do it use good parts or you are just gonna waste your time. really if your not planing to put and exhaust and do carb work your not gona have that storker motor run to its full potential.IMO try and stay as close to stock far as bore goes and if you feel the need bump up the compression a little. whats wrong with your motor that you dont rebuild yours?
Avid collector of useless things.