Thanks for comments. Re-looking at it today the gasket I thought were reusable probably aren't. They've got a small rubber bead that runs round each port. But holding the gasket flat none of the rubber seal appears to be proud of the service, therefore I can't see how it can seal. I expect they were compressed on the first use.
Also the gasket has inserted metal spaces where the bolts go through, therefore there is no way the gasket can compress.
Having done some more research on intake gaskets it looks like there are Vortec and non-Vortec ones, I'm wondering if we've got the wrong one (ours is a Vortec model). If we can get a paper gasket I expect this will compress better around the ports, will do some research tomorrow.
Nothing is ever simple !
'92 190 Prostar 260hp Indmar.