i had to make it whilst on the table with the gearbox in the " that looks about right " position . what gear box are you using ? i made a custom rear diff mount suit where i put the engine made custom mounts for both and gearbox then had the prop shortened to suit it all sits slightly off centre because i didnt want to cut n shut my driveshafts but it fits
wasnt easy and was a bit bum about face as i pretty much built the rolling chassis minus the seat brackets rad brackets and other gubbins now ive stripped it all off knowing it fits to complete the chassis before painting
as for the mouth my gearbox sits slightly into the engine bay to leave the gearstick where i want it #1 and the clearance of the gearbox at the top of the frame #2 and how much i had sticking out the bottom #3 them make sure you left enough room in the engine bay for the engine
:-)