Which model 240 did you try the crank pulley from? Your early 360 GLT will be fitted with a B19E engine, it could be that the 3 belt crank pulley you tried from a 240 was from a B200/230 series engined car and the crank is different.
I suspect you'll want to source a 3 belt crank pulley from a 240 equipped with a B21/23 engine and PAS - assuming they exist?
There are some aftermarket sources producing aftermarket lightweight crank pulleys if you're not able to source a good used example, for example here:
https://shop.klracing.se/sv/artiklar...-8v-3spar.html
Would be worth dropping them an email to check it will fit a 360, the only potential issue I could forsee is if the pulley is deeper than the original as there's less clearance between the pulley and radiator surround panel in a 360.