Bought the wrong tremolo -- 56mm spacing -- got a Mexican strat body w/ 52.5mm holes.

People are saying to fill the existing holes and re-drill, but I'm not confident. Could buy another bridge, but only difference would be hole spacing and would have to sell this one.

Could I widen (some of) the holes on the bridge horizontally using a drill or file, so it would be similar to a Wilkinson 5 + 1?

The two central holes could be used as-is, to locate the bridge 'horizonally' (if not both, then surely one could be used and it would make neglibible difference to the centring of the bridge). And the other holes could be widened 'horizontally' on the bridge itself, using a drill or metal file, so that the screws could fit in the existing holes in the body. These widened bridge holes would locate the bridge 'vertically' but not 'horizontally'.

Would this work?

('Horizontally' assuming the guitar is standing up, so perpendicular to the neck; 'vertically' being parallel to the neck -- not sure if there is correct terminology.)

you need to plug the holes and redrill, or get a correct bridge.

plugging the holes is really easy and will take all of a few minutes. Then you can install the bridge correctly instead of half-assed
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
Is there a reason why filing the bridge itself is half-assed? Why is filling the holes and redrilling preferable?
As long as you can use one of the holes as is, go ahead and widen the other 5 like the wilkensen. Just make sure that your reference hole is correctly spaced. I would pick a hole that looks close and then screw it in using that hole. Then run dental floss of fishing line through the tuners and over the saddle notches to emulate the e strings to determine if the bridge is correctly centered.

One argument against filling is that absent a really specialized plug cutter that I have never seen, the wood you would be filling would result in you screwing into endgrain which is not as strong.
Thank you so much for your answer, Rusty. It was bizarre reading all these forums and getting no talk about widening the bridge holes instead of plugging the body holes.

Another person said: "The problem with widening the holes in the bridge is that there is potential for movement and it'll look like a hack job". This guitar is meant to be a frankenstein, but I guess I will have to be very careful to extend the holes only in the horizontal direction, retaining the vertical dimensions exactly, so that the bridge still pivots correctly.

The two middle holes are VERY close, and both screw in without any pressure on the base plate. I will try the dental floss method but I don't doubt it will be suitably centred.

I also would prefer trying this method first, because if I buy another tremolo bridge I would have to sell the current one, and really I would only get $15 or $20 for it. I just have no confidence in my woodworking ability and if this doesn't work I will simply buy the correct part.
if you have access to a dremel, I would use one of the grinding stones. trying to drill metal when there is already a hole will be difficult. It will also be much smoother with a stone. if you don't have access to a dremel, maybe try wrapping sandpaper around a nail. I wouldn't go below 180 grit. just go slow.