Excellent build and I wish you every success with it!
Since I have a BVM F4 , same scale, but with AMT Pegasus power, about twice your thrust, a few points may help:
The BVM F4 tailpipes are exactly in line with scale datum, no extra downthrust needed and the model is not affected by pitch changes due to thrust, it is neutral. Mine also has the scale le slat of the J on the stab, and still not affected by jet efflux.
The C of G is 12 3/4 inches along the leading edge, from the root, as the LE is. 34 1/2 inches long so about 38 % along, beware, it is further forward than one might think !
Your model will be significantly lighter than mine, so I guess twin X45s will be perfect, they are quite brilliant engines, just installed mine in my BVM F16 ( original )
Good luck, I very much look forward to seeing it.
PS I have lots of F4 books and drawings inc the BVM ones if needed.