As other posts have said, CG position is the same wherever the engine is.
A downward vee tail does improve stability and control especially at low speeds/high angles of attack.
One way to get around the need for a long UC is to make it a tail-dragger and put skids/whells on the end of the fins. Example, the "Lazair" ultra-light. Other advantages of a pusher set-up are ; drag is lower, so speed is higher, (no slipstream along fuselage and over wing roots, tail surfaces etc ) and no yaw effects during take-off, so no side thrust on engine necessary.
This lack of prop-induced turbulence over fuselage and wing roots, tail etc is one reason why duct-fans and jets are "quick". The down-side is that lack of slip stream over tail surfaces reduces control effectiveness at low speeds, so larger areas, or movements, are required.