Sounds like you may have a slight combination of problems. First thing I would do is check the Battery and the cables, then scrape the terminals on the battery and the connectors themselves. Then get a heat shield (can't hurt...)

Then, I would do the starter relay mod. I installed (A ford starter relay) and I've not had one problem in 4.5 years with headers, the same starter and the same battery. And this year, the poor bird has seen maybe 500 miles, and has been sitting the rest of the time.
http://www.madelectrical.com/catalog/st-1.shtml for a complete kit with instructions only $30!

That's what I would do in any case...

Brett - 67 Drop Top 455!