I just redid my motor with a set of Trickflow heads and I bought a set of LT1 style centerbolt valve covers. The heads came from a guy I know and he did not have any problems with leaky valve covers while they were on his car.

What I noticed is there is a lip on the inner side of the valve cover that makes up the groove that the gasket sits in. On one side of the valve cover it extends down more than the other.

Now the valve cover will only go on one way based on the bolt pattern of the valve covers. So the deeper groove resides on the lower side of the head. I have tried 2 different valve covers and new gaskets with no luck. They all still leak at the lower back corner.

Does anyone have any ideas what may be happening?

 

I am running Trick Flows as well (gen 1)... Why don't you try removing the gasket and try placing the valve cover on the head to see if it fits down all around (the lip might be bent). One time I put my valve covers on with the red high-temp RTV only... I cleaned it well and put a bead in the groove, I let it set up till it was partially cured then set it on the head and snugged down the bolts.. if you wipe the surfaces clean and not use any degreaser it will come off easily in the future... this worked really well and never leaked.
I alway use rubber backed SS washers from the hardware store to seal the center-bolt holes.

 

i have never had a set of valvecovers that didnt leak unless some sort of sealant was used!

 

Make sure the lip isn't hitting inside the head gasket rail. Happened to me with factory covers and aftermarket heads.