Duster should have been a DEMON! Cranking issues!

Well it is finally assembled, registered and insured...if it would only be dependable! My 72 duster was a former drag car. It has a rear mounted battery, push button start and toggle switch ignition and fuel pump. It cranks slow or not at all. New 850 CCA battery. New cable ends to emergency shut down and from shutdown to starter relay and relay to starter. All ends soldered on. A .2 volt drop at starter relay and even less at starter! Put a jump she starts twice, third time, wuhn..wuhn...then, barely turns. Does not matter hot or cold engine. Do I need dual batteries? Jumping with my dually diesel which has 2 batteries and by 3rd crank....wuhn...wuhn..wuhn...wuhn
Engine grounded to subframe. Battery cable removed when parked, tested for draw - none, but remove anyway! I get 2.2 volts at starter engagement connection on relay, is that enough? If she don't fire by second crank attempt, jumping and waiting is the only way it will crank fast enough to light off! I am at wits end