A lot of times chain length is determined by your chassis. Does your J arm clear the right side seat strut when going through shifting movements? Moving motor to far forward, the shift linkage might interfere with this member. Another thing to consider is actual motor clearance to you, the driver. If the motor is too far forward, the driver will be hitting his elbow on parts of the motor. I agree, a longer chain would run cooler, so run as large rear sprocket as you can to get the gear ratio you are looking for.
#55 Honda CR80