Put the new cluster case next to the original (the one that fit) and compare them.
Either that, or take some measurements (of both boxes and compare)- at the points where you are tight.
Remember, only the bottom lip of the cluster housing goes behind the steel shell of the dash. The sides and top have a small flange (maybe 1/4") and that stays outside the steel . Only the body of the cluster case goes in the opening.
It is true that the opening is tight, especially in the lower LH corner.
It is also perfectly acceptable to bend, file or cut the opening to accept the cluster box.
Here is a picture - not exactly like real life but might help:
BTW - I just looked at what LMC is marketing as a replacement cluster carrier. Can't comment on dimensions but, the image they show doesn't look anything like an OEM unit.
Return the LMC one, get your money back and go to the junkyard. You can get an OEM instrument panel complete for probably $50.