Wheel Spacing

I just finished replacing all the suspension parts on my 65 Mustang (changing from 6 cyl to 8 cyl), including SSBC disc upgrade kit. I purchased the Scott Drake GT wheels (15x7) and have 205/65/15 tires mounted. When I look at the clearance, I found that the inside rim wheel weights and the tire come as close as possible, without actually rubbing the upper control arm.
It looks like the wheel backspacing or offset is wrong for this application (backspacing is stated as 3.75"), putting the inner rim too close to the control arm, although they are supposed to be for my car.
I have approx. 1/2" of bolt thread showing outside the wheel mounting hole. I can get 7/32" or 5/16" spacers. Is this what I should be using? Is the 5/16" and the resulting small amount of bolt thread OK? Or is there some other solution. (I don't want to send back the wheels, if at all possible.)  

just curious...  before anything gets returned...

are the wheels on the ground ... on some kind of turn tables.. to allow the suspension to be on the ground and the steering to freely rotate to check for clearance..

when the suspension is hanging.. there is less clearance to the upper control arm..


a big hint also..    you might with new wheels...  want to measure the inner wheel to outside of the caliper clearance... if you have more than 1/2 inch...   you can take the wheels back to the shop where you had the tires mounted and balanced..

have them use TAPE a weights along the center of the wheel.. so you don't have weights on the inner or outer.  most tire balancing machines have this ability...   although the TECHs may not all have a clue on how to get to that function..

i was in a small tire shop..   the owner of a jag would NOT let them stick weights to the outside of his polished wheels..

i stepped in.. grabbed the instructions from the back of the machine.. showed the tire guy how to set the controls for a single line of balance..  i then had to show the tire guy to NOT stack the weights up on top of each other or it would NOT clear the calipers..

i hope this helps..