904 and 8 3/4" gasket advice

An overfilled trans will blow the excess out the vent.....which is inside the bell house.

The rear seals inner seals will not blow out; there is no pressure in there. The oil level is below the axle tubes and the only way for oil to get up in there, besides over-filling is if you park the car on the side of a hill, with one side higher than the other.
But those inner seals do eventually wear out, but it is very uncommon.I have known them to go a quarter million miles
The outter seals fail more commonly and puke their grease into the brake housings. But these seals cannot be replaced except by removing the bearings first.
Cha-ching.