On my 68 the gasket (I found some NOS ones) is too small to sit in place while the nuts are tightened.
No big trick, just patience. Put the gasket over the studs and start the nuts on. Push the housing toward the valence an work the gasket in place as you tighten the nuts. Be patient you'll end up with the gasket in place and tight against the chrome housing!
My '64 1/2 vert. Ordered May '64. D code 4 speed, handling package, caspian blue, accent group, Ford blue manual top.
'68 vert. driver. Owned since '77. C code AT, AC, PS, P disc B, PT lime gold, standard black interior and top. NOS RF fender and left quarter.New top and folding glass.