Rob,

I've done this job about a dozen times - complete ball ache!
On my first build I had similar nightmares to you.

This build I've had to do it twice. Once when building the engine on a stand and it didn't leak. Then I had to remove the sump to check something so did it again and it didn't leak either (broke my heart removing my non leaking sump thinking it may never seal like that again but it did!)

This is what worked for me.

Genuine sump - wouldn't seal with a cheap non genuine one.

Obviously dry clean surfaces, use brake cleaner.

Sump on the deck I used a bead of sealer in each of the moons on the sump (similar to tiger seal)

Placed the gasket in and pressed the moons into location on the sump.

Then, thin bead of sealer on the gasket moons which would mate to the 'block'.
Offer up the sump carefully and check both moons are snug and the lengths of gasket fit evenly.

then, just pinched up the bolts uniformly working center out diagonally.

then I went round again a couple of times nipping them all up with a small 1/4 drive. I didn't use the torque settings as these never worked for me.

Thats what I have done and its worked successfully for me. The test is when you start running decent boost and mine has been leak free with 26psi up it.

Good luck, I know how frustrating it is.

Dan