I have never had that problem when I have replaced boosters. However, there are at least two different boosters and, as I recall, there are two different "shaft" lengths behind the booster. I think some aftermarkets come with two different shafts and you install the correct one.
I am vague on this, but I know a spent some time trying to verify that I was getting the correct booster for my 83 C20. I would double check the Rock Auto and Delco catalogs online to make sure you have the correct booster. I use
www.oehq.com and
www.oewarehouse.com to look for Delco parts.
Personally, I would use only Delco for a functioning safety part like a booster. For the rest of the brake system, I use Delco for the wheel cylinders and calipers if the discount price is reasonable, and use aftermarket for the drums, shoes, pads and other strictly mechanical parts. But each to his or her own.