Hi Thomas,
I have ran in to this type of problem before and I think you are on the right track as far as grounding the system goes.
I believe that in 73, a second or primary starter relay or solenoid was required. I ended up replacing the solenoi...
what i would do is go trough and check EVERY WIRE i know it seems long and unesasery but do it,you may have a lose wire. Also you may not have the right guage of wire or the right battary .
sskiller is right. you need a ground wire from the engine to the body to complete the circuit. it goes battery to engine, engine to frame and engine to body( even if it's a unibody). hope this helps.
Hey Thomas! The comment about useing a amp meter to check your starter draw was right on and cranking the engine without the plugs will also tell you alot.With an engine as motified as yours the starter may not have enough horsepower to overcome the compression it's making.The answer may be a higher torque/racing starter.Good luck!