is my car in any way special/rare or more valuable than any other? 70 340.

First of all, Welcome aboard. You mentioned it was originally a manual but a 4 speed has been swapped in, do you mean the car came from the factory with a three speed ? So you have an FY1, 3 speed 340 Swinger with the matching number block and you want to customize it ? Obviously it is your car and you can do what you want, however, when it come time to sell you are cutting your market down considerably. Post some pictures so we can see what you are working with.