With the higher compression engines this is a common problem. But I'm inclined to ask about the starter too. If it is too close to the exhaust, it will heat up and need more juice to crank over the engine which is hot, expanded and tougher to start. Had a similar issue with a Vette. The gear reduction starter with a lower profile housing worked great. The smaller housing keeps it farther away from the headers, so it stays cooler, the gear reduction, just makes thing easier for the starter and the battery.
What type of starter are you running? If this isn't it, then like people said above, cables, ground and bigger battery may be needed.