IMO they actually seal pretty easy, at least mine have never shown any signs of leaking (which is more than I can say for the crank seal). Wellseal will no doubt work fine, but I've always used Permatex #3.
However, I have heard that some gasket sets don't have enough felt in them. You should have so much felt that you can't pack it all into the holes even with a drift & hammer (after soaking in your choice of sealer) and have to trim the end off.
BFE used to sell extra felt, or you could probably find it at an industrial supply place, like MMC.
PS, I've never bothered with the square drift shown in the manual. My drift is a chunk of aluminum rod from the scrap box that just fits into the hole.