Good oil pan gasket?,

Felpro sells a gasket set for 351C that works well. It is four pieces for the oil pan and it has worked correctly for me on at least three occasions. Most of us put a dab of RTV in the four corners where the pieces meet. The set includes many other gaskets including head gaskets, timing cover, water pump. valve covers, rear main, thermostat, etc. It does not include usable intake gaskets. You get a lot of gaskets for your money. Here it is at Summit Racing:
https://www.summitracing.com/parts/fel-fs8347pt

Rock Auto has a selection of the oil pan gasket only, both Fel-Pro and Mahle. Both are cork/rubber gasket.
Not aware of a one piece. Both the aforementioned are 4 piece with a little RTV where they are joined together.

there is no 1 piece oil pan gasket available for the 351C