The last production version of the Merlin was the 13x series which came in the four different combinations of left-and right-handed and high- and low-altitude versions for the DH Hornet. So this or a derivative of it, which was no doubt on the drawing board, would have been in mind.
Its principal advantage would have been that it worked, since the assumption is that the Griffon variant had not and would not.
The Merlin's upper profile is higher, so maintaining the pilot's view over the nose would have meant either mounting it lower so the nose bulged down underneath, or raising the cockpit unnecessarily high for the Griffon. Its thrust line is also higher, affecting the pitch trim change with power, and stalling characteristics among other things. This would have been reduced if the Merlin were lowered, otherwise pilots would have just got used to it; the difference was not a major issue with the Spitfire's handling. The final solution would have affected the mximum diameter of the propeller to maintain ground clearance, an issue which on the Spit led to the Griffons' growing an extra blade to make up for their smaller diameter.
The Merlin also had a higher specific power output in terms of bang per pound weight than the Griffon and consequently was not as heavy, so it would have needed balancing in pitch. If that was done by moving it forwards a little, then this would make the nose longer and the tail fin would also need to be larger, i.e. oversize for the Griffon. Alternatively, the Spitfire made significant use of counterweights in the tail to accommodate all its various engines; but since the Spit was designed for the Merlin and later adapted to the Griffon, while the Spiteful was the other way round, the ingots would go in the nose with the Merlin.
Then of course there are simple physical things like provision for fitting a different mounting frame, for any extra service lines through the firewall and space allowances for varying ancillary items.
But all that would not have been worked out in detail, it would just have been in the back of the designer's mind, to try and not rule out the Merlin conversion altogether.
