There is a good resource here:
Chrysler, Dodge, and Plymouth VIN Decoder 1959-2005
For '70-'74 Challengers, your VIN will look something like this:
JS23R0B100001, with the first 7 characters denoting what the car is and the 100001 (or whatever you have) is the actual sequence or serial # of the car
1st character: Always 'J' for Challenger
2nd character: 'H' for 'High price class' (non R/T Challengers) or 'S' for Special (R/T and rallye)
3rd & 4th characters: '23'= 2 door hardtop, '27'= 2 door convertible, '29' was only used for 1970 R/T or R/T SE models
5th character: engine code. B=198 slant six, C=225 slant six, G=318 2bbl v8, H=340-4bbl v8, J=340 six pack v8 (Challenger T/A only), L=383-2bbl v8, N=383 4bbl v8 (standard equipment on R/T), R=426-8bbl Hemi v8, U=440-4bbl v8 and V=440 six pack v8. L also= 360-4bbl on 1974 Challengers as the 383 was gone after '71
6th character: number representing the model year '0'=1970, '1'=1971 etc
7th character: assembly plant code. B=Hamtramck, MI and E=Los Angeles, CA
Hope this helps