I have 81 f250 4x4 with 351m to 400 conversion.
I currently have an oil leak due to taking off the pan too many times.
I was looking to do a 1 piece gasket, but can oil find a 1 piece gasket for the Windsor. So the question is will that fit on my 351m/400? Or do some of yall know where I can get a 1 piece for the 351m/400?

 

Why not use a bead of RTV instead of a gasket.
Most new engines don't use gaskets anymore. Are you thinking you will need to take the pan off again (or several more times)?

The downside to RTV, is that both surfaces need to be clean (no oil on pan/block surfaces) or it won't seal.

 

Those are two different engine families, I am sure they won't fit.

Problem with using rtv, the end seals can be pretty thick and hard to seal with rtv.

If you are doing this in the truck, this is a delicate operation. There have been more than one person on this board doing this in the truck and then end up with no oil pressure because of bits of gasket falling into the pan. It's very difficult to do this in the truck.

I would very carefully try to get ALL the old gasket off without it falling in the pan. I would then use a combination of glue and rtv. I would buy that yellow type sealer they used to sell, or just buy some trim adhesive and glue the gasket to the engine block. I would also put dabs of rtv in the 4 corners where the gaskets mate up. Once it's all in place on the engine, then put the pan back on.

 

No, it will not fit - they are completely different engines with very different dimensions. The guys are giving you good advice. This is a very painful job in the truck, and you need to do it very carefully or you will have to do it again.