Such problems are (well almost) always ground issues. I replaced a battery and went through a starter and a relay with what you described. Does it feel like the starter is fighting like: struuuuugle turn! struggle turn turn struggle spin spin spin. This turned out for me to be the attachment point from the cable to the starter. A bit of grunge had gotten on the terminal. Thats all it took. Sand the cables on boths side till they gleam then sand the terminals til they gleam. Use fast orange (the hand cleaner): it will take off grease like you wouldnt believe. Do the same to the relay-to-battery cables.
The reason I say is that I used the flimsy flimsy battery from the corolla to boost my stang on multiple occasions. If you think it IS the battery, then you can resolve the problem like this. Connect 2 batteries IN PARRALLEL using jumpers then jumper this setup to the terminals. This will start anything. If still no dice, it's the connection. WARNING. If there is a short, after like 5-6 seconds, the cables will start to smoke!!
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91 GT Carb conversion, holley 600 double pumper, edelbrock performer intake, FMS "C" drop springs, march 1000 underdrive pullies, crane 1.7 roller rockers, GT-40P headers, bassani x pipe, american thunder catback, FMS 4.56's, msd aL6, trunkmount battery, A/C eliminator kit, 3000 stall tci streetfighter, AOD with transgo kit, A+ servo, 300M hardened lockup shaft, kevlar bands and 28,000 gvw trans cooler, 3 core radiator, 300 lbs stripped with a full interior
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