Neither Winchester, nor anyone else offered the .45 Colt in their leverguns, UNTIL modern times. The reason usually given is that the gunmakers in the 1870s did not think the .45 Colt rim was "robust" enough and would give problems in a lever gun. The .44-40 actually has a LARGER diameter rim than the .45 Colt, and was the original chambering for the 73 Winchester.
Modern .45 Colt brass has a stronger, thicker rim than the original cases, and does seem to work well enough in lever guns today.
By the specs the .45 Colt LOADED length can be 0.008" longer than the .44-40. This small difference has no effect on loading the rifle. .45 Colt rim is .512", the .44-40 is .525" (max specs).
IF your repro 73 rifle is in spec the small difference in size between the .44-40 and the .45 Colt should not cause any difficulty.
If you are dragging the rim on the receiver, I think its probably something you are doing, not the design of the gun. That being said, its a guess, because I don't know if your REPRODUCTION 1873 rifle is actually made to the same dimensions as the original, or just looks like it is...
One thing I would not do is monkey with the gun. At least not until ALL other possible options failed. And, If you're gonna get springs modified, get REPLACEMENT parts and modify THEM, keeping the factory original parts original. That way, if your modification doesn't work, the gun is easily returned to "normal".
Good Luck.