DIY acapella help… i have an old vinyl im trying to extrac

i have an old vinyl i want to make an acapella from. it contains the original mix 5mins 40secs, and an instr-umental tmin 07secs. is it possible to extract the vocals? i was told both tracks must be the same length to work.

AND IM FINDING IT IMPOSSIBLE TO FIND ANY ORIGINAL WITH AN instr-umental OF THE SAME LENGTH.

please help…

cheers!