I have a 1970 Delta 88 (front disc, rear drums). The brake booster just went bad. Looking at local auto stores, eBay, Rock Auto, Summit Racing, and etc. they all recommend a different looking brake booster that doesn't look it would fit or line my master cylinder up with the brake lines at all. Mine is 6" deep and the others look much less.

Here are a couple pics of the brake booster I have and need.

 

What about getting it rebuilt? I am not sure who does it but I do know there are companies that do rebuild them. The master cylinder looks like a drum brake front and back unit to me. Disc brake masters have a much larger reservoir for the front brakes not both the same.
Hollander interchange manual shows a 70 88 disc brake booster to be the same as 69 disc brake as well. But B body not A body and not 98 in 70

 

apparently I found out that on the cars with front disc, it's the newer 1971 style brake booster.