with the centre beam sheeting do i need to get an opening stitched into the tramp for the pulley/cleat?
Yes if you are mounting a swivel/cleat.
If you just want a ratchet block you only need either a tab stitched on (a lot of A-classes seem to have this) or an eyelet punched in either side of the beam - then you just tie a loop of rope around the beam and attach the ratchet block to that.
