Oil pressure light keeps coming on.

[Deathtrapgarage]

i have a 1962 c-10 with the 235 c.i. inline 6, recently i changed the oil which hadnt been done in a long time i assume due to the thickness and color and now my oil pressure light has been coming on, i believe i put enough oil in it, the only other thing i could think if it has the correct amount of oil would be the oil pump? any ideas? ive been thinking of putting a pressure gauge to check, any body have a good spot to put one on it and where is the oil pump located? new to this truck and dont have a manuel, also how many quarts do these trucks normally take? thanks alot for the help.