I have a 67 chevelle with power brakes, disc on front....wanting to make sure that I don't buy the wrong corvette rally wheels.

First, are the lugs 4 1/2 inches apart or 4 3/4, I've heard both? I've measured but it's not that different to be sure.

What is the space needed on the inside/backside of the wheel?

Thanks for your help

4-3/4" is the bolt pattern
A stock backspacing will be about 3-1/2" on a 5" wheel
you would probably end up with a 3-1/2" backspacing on any wheel up to 7"

Welcome, Nathan
The correct disc brake option for 67-68 Chevelle (RPO J52) used a four piston caliper. If this option was requested, it would come with 14" rally wheels. If you want to run 15's you have lots of options.
The highest prices paid are for desirable codes for specific years and cars. The wheel will attract more interest if it’s freshly blasted and correctly painted or ready for paint. The numbers that matter here are the ones out by the valve stem. These numbers indicate when the wheel was assembled and its usage.
The basic way to identify a wheel: There is usually a 3-digit set of numbers and letters to the right of the valve stem hole, such as K19 k17 k13. Those are decoded as; K=Kelsey Hayes the wheel manufacturer, 1= Chevrolet motor division, the last number is year of manufacture 9=69 7= 67 3= 73. To identify the month and day, usually on each side of the valve stem will be a number 1 thru 12 on the left and a 1 thru 31 on the right followed by 2 letters such as AZ or FW or YH, ect. There are lots of two digit letter codes.
For example: an early dated YH rim (15X7 69 Camaro Z28) is vary desirable. The same size 15X7 with a FW code (used on many models and years) has little value.
Check your codes, post them here, and you’ll get a more accurate answer in regards to value.