I'd go with the 18X8 prop. As w8ye said 7200rpm is a good rpm for this engine. As for the weight factor, you're only adding 40 grams to the total weight correct? That is only 1.4 ounces. The plane should not fly any different with the exception of using a slower turning gas engine. You could try moving the rx battery towards the back to shift some weight if you want. What prop were you using on the 1.08?