what is the correct oil wieght for a 77 ford f100 with a 351 c

 

about 4 pounds

 

I thought that was for a 351M?

 

That's a tad light, 5 quarts would equal about 7lbs.

 

To actually answer the OP's question. If the engine is in good shape, use 10W30. If it's worn use a 20W40 or, if really worn, straight 50 weight.
Also, be sure to use an oil that has adequate amount of zinc (ZDDP) to help protect the cam and lifters, although this is less important on high mileage engines.

 

I was thinking synthetic