hey, what's keeping the engine from not fitting???
be more specific, no offense but no one can help you if you don't list specifics!
truthfully you should buy the traxxas big block conversion, because it is meant to fit and meant to fit the engine right, you should go with rear exhaust, because that means a shorter header which means more power.
also, you may want to find a roto-start conversion or just put the dynamite spin start on it!
i would personally go with the spin start. my lst fires right up with the spin start. also try to find a smaller engine for the revo like a decent .21 (OFNA PICCO .21 or .26 MAX) those are meant for traxxas's, it will save you lots of money in the long run, because that .26 will cause you to break stuff, meaning you have to go buy more upgrades, if you are sure on big block go with it.
but if not go with a high end small block like the o.s. .18TM, that engine is meant for the revo and has plenty enough power