You pulled your old inserts out first, measured them, and then bought something that is 15 mm longer?
Sometimes, in the bottom of the strut tube, when changing from wetlegs to inserts, fitters use a stack of spacer washers to take up any slack under the big top nut( gland nut?)
Pick up your strut, tip upside down and shake/bang it, if any thete they should drop out.
Smart fitters will use racetape to hold the washer stack onto bottom of insert.
Some inserts have an obviuos knob, lump of protuberance, that can be docked off, some up to 20 mm long, this is done when a shock insert manufacturer makes a single unit that can be multi fitted to various cars, by trimming the "knob"
Do NOT under any circumstance cut the insert tube, it is under high pressure, and when you break through with a hacksaw the hole is miniscule, and becomes an extreme high pressure jet of metal filings and oil.
The rear insert lengths may be difderent to fronts, dismantle and trial fit.
FWIW in future get absolute confirmation that what you are buying is for your car.
The inserts ypu got could be for a Massey Ferguson tractor for all you know, and could be dangerous to use, as it is not valved for your car.
Generally, buy inserts as a set of four from one manufacturer.
Bloke i race against was having a big whinge about what his car is doing under cornering, i adked him about suspension set up, he tells me he has Bilsteins gas in front, and Koni oils in rear, and could not understand concept of gas pressure vs non gas pressure on foaming oil in race use. Either or, but not a combe !!