This backspacing doesn't sound right!!!

So I have a 1973 swing and I have had slicks on it for over a year now and decided to bite the bullet and order some new wheels so I can put my street tires on. The wheels that are on the slicks were orginialy on the street tires so I figured I could measure them and use that backspacing to order my new wheels. The backspacing on them is only 3.5 which seemed small to me. The tires are 235/60/15 and I thought I would ask around on here to make sure I am not screwing this home somehow. Thank you in advance for the input.