The gasket looks squashed out which means over tightening. The original bolts in a GMC had a Phillips head as well as a hex. They were meant to be tightened with a screw driver only. Chev may have had slotted screw driver heads. Before I install a push rod cover, I put as straight edge to the cover and see if the sealing ridge is straight. I also check the bolt holes. If they are not flat, I lay a small ball pein hammer in the hole and tap it until the hole is straight. Most covers are bowed from being overtightened at some time in their life. Straightening them is a rather tedious job, but it can be done. I prefer to use a sealer that becomes rather hard such as Permatex Super high tack #97 against the block. I then use screws to locate the gasket the gasket until it is stuck to the block. I then apply a sealer that remains flexible like Permatex No 3 on the cover and the gasket. I then remove the bolts, put the cover in place and tighten the screws with a screw driver or nut driver. The vent tube is a press fit and probably leaks because it is distorted a little. I think you need to pull it out and try again. What you are doing now is like trying to patch a hole in a boat from the inside of the boat with bubble gum. I would pull it back out, clean the two mating surfaces absolutely clean and apply something like Permatex no 1 to both surfaces. http://www.permatex.com/products/Automotive/automotive_gasketing/gasket_sealants.htm