1975 Chevy C6500 charging problem

ok so i have a C6500 however it is a cab & chassis with a water tank. I recently replaced the atlernator & starter. The engine is at least a 454 maybe even a 502, The battery stoped charging....

So maybe its the battery........nope
I put in my blue top optima battery in, out of my boat.....it worked for awhile....like 2 days
so on my elcamino   you have a stater w/ selondin on i....and the alternator...and battery...
with fords, like my mustang... it has no selnoid on the starter so it has a selnoid on the firewall....
and here is way i mentioned that....upon inpection of the 1975 c6500 there is a selnoid on the starter.....BUT it ALSO has a selnoid on the firewall......
So is there a need to hav that selnoid if there is one alreadly on the starter.
PLEASE Help me !

Thanks in advance

have you tired to have the alternator tested?