Not safe.
The line capacitors in the PSU can hold their charge for days. 99.999% of the time, a jolt from that won't kill you, but you don't want to be that 0.001%
cutting the wires from the outside ensures absolute safety.
Anybody that needs to ask how to mod a PSU doesn't have enough experience to do it from the inside safely.
Go from the outside. absolutely safe.
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