There is more involved than just wheel width; the offset is important too. But since I can squeak by with 205/55 tires on 5.5" TR6 rims under my stock TR3 body, I don't think you're likely to have a problem with the width of 6.50's. I do question how old they are, though, unless you bought reproductions from Coker or similar.
Practically all modern tires are lower profile than the old bias-ply tires were. The profile is identified by the second number, which is (roughly) the sidewall height expressed as a percentage of the tread width. So, for example, a 185/65 tire will have a side height of 185 * .65 = 120mm.
185/65-15 tires are very slightly shorter than the stock bias ply tires, but the speedo error is only about 4%, or 2.5 mph @ 70 mph. A good choice, IMO, and a perfect fit for 5.5" rims. I actually tried some once, mounted on the original 4.5" rims, and they worked out well for me.