[sb85Berlinetta]

i just got my car back from a 350 swap...my car has been sitting in cold wet weather and leaks from my T-tops...my battery keeps going dead so i take it to auto stupid and they do there lil test on my battery and they say its fine....I go to work and on my way home my diggy gauges start flickering or when i turn my blinkers on my power shuts off but my car still runs....the next day i take my battery back up to auto stupid and they test and all i put it back in and hook it up car starts....i go get my alernater tested and they pick up no volts so i take it off and get another test done it says it's no good...i get a new one and hook it up car starts right up no problem for alil bit and soon as it turns dark i get the same problem....what could it be and what will i have to do to fix it??