C20 brake conversion question

[BigDwa]

Howdy, first time working on a disc brake conversion and I have been reading through many of the threads. I have a 1969 Chevy C20.

I am leaning towards the Performance Online C20 8 lug disc conversion kit. I want to use the 2 1/2 drop spindles when I make the disc conversion.
However, there is a note that I am having a hard time understanding. The note is: Note: May not work with factory 15" drum brake wheels. Moves wheels out .75", 3/4" on each side

I do have 15" wheels on the truck currently. Don't believe they are stock rims as they are not steel rims but aluminum. Would prefer not to replace rims at this time if possible. What do I need to check? Anyone else used this kit before and if so did you run into anything unexpected?

Link to the Performance Online disc conversion Kit: https://www.performanceonline.com/19...-8-Lug-Deluxe/

Thanks in advance.. David A.