Best gasket sealant for overdrives?

[hypoid53]

I have recently rebuilt a D type overdrive and 3 rail gearbox for my GT6. As I am not ready to fit it back into the car, I filled it with hypoid and stored it horizontal. After 4 months I was getting a weep of oil at the bottom of the mating flange with the gearbox adaptor to overdrive unit. I used the correct gasket and plenty of 'Wellseal' (which is recommended in the workshop manual) and it still leaks under gravity!
I have now removed the unit and cleaned up the mating flanges ready for reassembly.
Has anyone experience of a really good gasket sealant that works...especially in the environment of an overdrive, ie. Hermetite red or some such?

[thealligator]

BLUE HYLOMAR or Evo stick.