I would try doubling your toe-in to 1/8 inch. Since you are using original weight engine/transmission, you may have too much castor. Is there a reason why a lowered axle needs twice as much castor as original? One more question, what kind of tires are you running? Are they new?
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission