The Merlin appears more compact and looks like a much better overall design as far as fitting it into an aircraft, the Allison on the other hand has it's cam drive gearbox's? outside the profile of the engine and it's intake manifold looks convoluted with plenty of choke points.
The Allison intakes are equal-length units. The Merlin intakes aren't. So your observation there isn't quite inline with real life. The Merlin is not really much more compact in general. They are substantially the same size.
The dry weight of a V-1710-89/91 is 1,350 lb. Later Allison were closer to 1,450 lb. A Allison F-30R (which is a V-1710-111) will fit inside a box 29.3 in wide, 37.6 in tall, and 86 in long.
The dry weight of a Merlin 66 is 1,645 lb, but that is a 2-stage engine. A Merlin XX ws 1,420 lb. Much more comparable to the Allison in that regard. Later Merlins were 1650 lb. A Merlin XX fit inside a box 29.8 in wide, 41.2 in tall, and 70.6 in long.
It's a rare aircraft when 16 inches make a difference. The engine mounts are usually pushed farther forward than that for CG purposes. Radial engine mounts are sometimes several FEET longer for CG purposes. They had NO trouble converting from an Allison to a Merlin or even back again. The Allison would have to be moved forward anyway since it was generally a bit lighter than a Merlin.
So, the Allison and Merlin, both single stage, are almost exactly equal in width (within 1.7%), The Allison is 9% less tall than a Merlin, and 18% longer than a Merlin. One is an updraft carb and one has a downdraft carb. The Allison, if viewed from above looks busier because you can see the carb and distributors. The Merlin, if viewed from below, looks busier because you can see the same carb, etc. on the Merlin from below.
The Allison was a bit lighter, dry weight-wise. Later, the 2-stage Allison was maybe 80 lb heavier than the Merlin, likely due to the second stage being external to the engine. The wheelcase had some weight. Early to mid-war, the Merlin usually put out slightly more hp and had slightly better altitude performance. Once the 2-stage 60-series Merlins showed up, the hp increased slightly, but the altitude performance really jumped, versus the single-stage Allisons. Not so for the Turbosupercharged units in the P-38 or the later aux-stage Allisons. Late-war, they were well matched, hp and altitude-wise. The late Allison never got quite sorted because jets were higher on the priority list than pistons were. I have personally seen a G-series Allison putting out some 2,200 hp race at Reno without issues because the guy who built it, Joe Yancey, knows what he is doing with any piston engine. Of course, it also wasn't running at racing speeds at 30,000 feet, either. But it WAS running at racing speeds at Reno and won Bronze, won Silver, and then went back to the standard Allison for the Gold race because they weren't going to run faster than 430 mph or so in what was a 485 mph race anyway. A lot of the deficiency was that fact that the particular Yak-9 airframe in question wasn't optimized for speed like Strega and Voodoo were and are. That makes a difference that is difficult to quantize because of the large power difference between a stock P-51D and either Strega's or Voodoo's airframes. How much speed delta is due to power and how much to aerodynamics? I personally can't say, but I suspect the power difference is the main contributor to the speed difference. A stock P-51D makes 1,490 hp or so. Either Strega or Voodoo likely make close to 3,850 hp.
If you use the old cube root of the ratio of the power delta and use 1,490 hp and 360 mph (stock P-51D performance region) and 3,850 hp for the new power with no other changes, you get 494 mph for the new speeds, which is very close to reality. That's what makes me think it is mostly power than makes the difference.
So, anyway a sorted G-series unit is a known real possibility since it was DONE recently, and that didn't quite really happen during the war.
All for now. Cheers.