I assume you mean the opposite? You would have to slow the .303 down considerably to get the same velocity as the .375 2.5”. The standard .375 2.5” nitro load was around 1950 FPS. That is a very anemic 270 gr load. Modern powders rarely make any meaningful difference with respect to velocity. Indeed, modern load recipes are often a bit slower than the original nitro loads due to modern liability concerns and lower safe pressure thresholds. I can’t think of any animal I would prefer to shoot with a sub- 2000 FPS load that I wouldn’t prefer to take on with a tough 180 gr bullet moving at 2500 FPS plus from a .303. And yes it would be similar to the .375 Winchester that had a mercifully short lifespan for similar reasons.
With respect to getting an old Lee Speed or rarer yet, double up and running again for the fun of it? Sure, why not. But with respect to capability, it is what it is.