1970 challenger 340 6 pack TA

Is there a trim tag?

Yes, on the inner fender near the battery.

What should be the code on the engine?

E55 on the tag, also look for A53. The VIN should have a J as the 5th digit.

Are there any body stampings on the body I need to verify?

The radiator support & cowl should have partial VINs if they are original to the car.

Trans codes?

D32 for automatic & D21 for 4 speed.

Rear axle codes?

Not coded on the tag, but they all came with 3. 55 or 3.91 suregrip rear ends. Check the window sticker / broadcast sheet 'D' codes to know which one.