By using the blocks you are essentially creating 'stand offs' the use of which is common to allow for different engines. You are actually changing the length of the moment between the CG and the prop. But if you leave the CG in the recommended position you should be fine. I've done this on 2 planes which are British kits in order to install 4-strokes rather than the designed for 2 stokes and they both fly perfectly.... but I'm not a 3D pilot. So perhaps one of the aerodynamics gurus will chime in. Your other choice would have been to recess the engine into the firewall so as to leave the prop in the same position but I suspect it's too late for that now. And that alternative does not help the tailheavy problem.

Definitely wait for some other responses. But my anecdotal experience on this 1/2" modification says it's ok.