The 777 wont fit the 1/8th scale B-25, nor the 1/7th scale. If they would fit 1/7th I would consider them for the P-61. The diameter of the 777 is 9.1, the cowl on the 1/7th B-25 is 8 -9 ", so it wont fit.
It would have to be 1/5th scale to fit properly, and at that size, It is more than likely nearing the 100 pound range.