it may not be your intake gasket. '85 would have a computer (unless you removed it...) possibly it could cause a problem? i dont know anything about vehicle computers, but it might be something to check into.
now, for your question...
'81 and up 258's should have separate intake and exhaust manifolds/gaskets. mine is an '81, and it does.
yes, you should be able to change the intake without removing the exhaust. the exhaust is bolted to the block with (4?) bolts/nuts, and the intake seats between the ports of the exhaust, using bolts with washers that seat on a metal lip cast into the intake. there should be (8?) of these. as a side note, when you replace the gasket, you probably would want to replace these with new, grade 8 bolts(+antiseize compound). forgive me if this description does not make sense, when you look at it, you will see what i mean. to get some of the bolts out, you will have to use some u-joint ratchet extensions, this is fun...
PVC? i am supposing you mean RTV, this would not be good (or necessary) to use on the intake . when you start the engine after charging the gasket, it will smoke a little, this is the gasket heating up and sealing itself.
if you buy the gasket at a place like autozone, it will probably come with both intake and exhaust gaskets, + a little o-ring and gasket for the manifold heater. if you into the motor that far, you might as well change both while your at it.
i hope that i have been a help, good luck!