[bobcowan]

Usually caused by not enough amps getting to the starter. Could be lots of reasons for that - like a bad battery. First, take the battery down to Autozone and have it load tested.

Check all your cables. Make sure all the connections are tight and clean. Be sure and look under the insulation, corrosion can get in there.

What kind of cables do you have? Did you use welding cable? If so, get rid of it. Are they at least a 0 gauge? With a big block, if the battery is in the trunk, I'd use a 00 or 000 gauge.

Are you using a standard Ford starter relay? You might want to replace that.