Wheels And Tires

My son and I are trying to revive a 65 couple with an I6, manual steering, automatic transmission and drum brakes. We want to convert the front to disc brakes and stay with the 4x114.3 4 lug pattern. We will likely use Scarebird to do this.

We have been doing our research and have found a table that says we can use wheels that are 15x7 with a 4.25 backspace and tires that are P225/60R15.
http://www.allfordmustangs.com/photopost/showphoto.php?photo=33125

So we are thinking we will use wheels that are 15x7 with a 0 offset and 4 inch backspace. For the tires we will use the recommended P225/60R15 from the chart. So we are spot on the recommendations with the exception of the backspace, which is 4 vs the recommended 4.25. I have not been able to find the 15x7 with a 4.25 backspace.

I am wondering if the difference in the backspace is going to cause a problem. It is only .25 inch so I think it should be fine, but would really like someone to either confirm my thoughts are correct. Or tell me I am wrong and what I should do differently. Any help and guidance would be much appreciated.

I was told that if I go to a 15x7 wheel I will lose a substantial amount of power. Has anyone had experience with 15x7 wheels and a substantial loss of power?

rbohm, thank you for the input. I wish I understood it like you do.

It makes sense that the wheels alone will not make an issue if compensation is made in the tires. So if I use 15x7 wheels and tires that are P225/60R15, will I be in the right ballpark? Reminder, I have an I6 and do not plan to make a change. I guess the bottom line is, if I use the setup I listed, will it create a measureable decrease in power and/or will it mess with anything else like the gears, speedo, or anything else?

Thank you again for the help everyone.

We have never had this car running, we are trying to get it up and running. The tires that came on it are 205/70-14s. I am hoping you can tell me what the transition might do if we went to the P225/60R15. Thank you again for your help.

rbohm, I know this is going to make you shake your head and wonder how I think I can accomplish the task or bringing this car back to life but... I am not sure what you mean by that last comment. I think you mean it will work, but want to make sure. And do you think that set up with a 4 inch backspace will work? The table I saw said I should use a 4.25" backspace with those wheels and tires. Will .25" make a big enough difference to worry about?