The "C" and "R" models have a different size tank. I can't find the specs for the "early" VTX1800R,S models, but the rest are below.
External fuel pump:
VTX1800R(S,T,N also)-5.3 gal
Early models wtih fuel pump in the tank:
2002 VTX1800C - V&H Bigshots w/baffles removed, Power Commander dyno tuned, 200 rear tire, Hardstreet Slim Bags, Palladin removeable sissy bar, Aeromach pullback risers, etc.
2003 VT600CD VLX - Holesaw exhaust mod, jetted appropriately and that's about all, but not for long......
1995 VT1100C2 ACE - V&H Straight Shots, jetted appropriately, 12" Mini apes, Saddlemen Teardrop Saddlebags
The mods are never 'done'....