John, welcome to VAF.
As to your question, here are a few things to consider.
I would think a 7 would be a better start than an 8, due to fuse length.
A rear engine is going to raise havoc with the C/G, and the shorter fuse of a SBS will mitigate some of that. You will still need to extend the front engine mount, I would imagine.
The wing structure will have to be re-engineered to carry the additional weight, the additional load of the tail boom attachment, the additional fuel capacity ETC.
The fuse will need to be re-engineered to carry the additional weight, the additional vibration issues will have to be analyzed and dealt with, and of course the landing gear will need a total re-design.
The tail volume will most likely not work with the reworked balance of the aircraft, and will need to be re-engineered also.
Not much RV left by the time you get done, is there?
Anyway, I am not an engineer, so my thoughts are only a SWAG, but you get the idea. Pretty sure I have missed a few things.
Thanks for the chance to do a bit of mental exercise
As I said before, welcome aboard.