guys this is how i do cvh sump gaskets and ive never ever had one leak one drop of oil i swear!
firstly make sure the block surface is clean and oil free,same with the sump,wipe with thinners if possible to make sure its not oily, then i use a spray glue called "trim tack" to stick the gasket to the block! DONT try sticking it on the sump youl never do it.
then give it a few minutes to dry and re- fit the sump tightening the bolts just enough untill they go tight. dont go mad or youl snap the bolts but dont worry about tightening them up properly,you wont squash the gasket,thats why theres metal studs on the face of the sump.
DONT use any kind of sealant or silicone on rubber gaskets it makes the rubber slippy on the metal and will cause the gasket to spread instead of gripping the metal and sealing!

NOTE before fitting the sump back on make sure its not distorted or has been real over tightened, you can tell by looking at the holes,they should be flat round the holes and not bevelled in.
i have also seen a fair few cvh sumps rot and leak out the actual metal (side) of the sump,so check for this too.