We have previously proposed molecular circuits designed from polyaniline polymer strands, polyacetylene polymer strands and charge transfer salts acting as transistors. Due to unique properties that are demonstrated in this manuscript, we propose the use of carbon single wall nanotubes and transition metal endohedrally doped single wall carbon nanotubes (SWNTs) for utilization in molecular electronics. Different transition metals were used in a systematic fashion to manipulate the molecular orbital energy gap (HOMO-LUMO gap) of metallic (Ch = (n = m)) nanotubes. Gradient corrected, Density Functional Theory (DFT) Self Consistent Field (SCF) calculations were used to calculate molecular orbital energy levels, HOMO-LUMO gaps, electron affinities, ionization energies and other electronic properties for these molecules. The effect that a SWNT's length has on its HOMO-LUMO gap was investigated. DFT-SCF calculations were also used to demonstrate how multiple metal filled nanotubes could be used to construct a molecular nanotube based transistor.