will disc/spindle swap take the 3/4 out of 3/4ton?

[nightwrencher]

Hi, new to the board here. I have a 66 C20 I put together to tow my firebird to the race track. I love the truck, but those 8-lug manual drums w/ 16.5" rims leave a little more to be desired. I already plumbed a '70 dual master cylinder setup in, but I know a guy who has a disc brake/suspension setup still bolted together he'll sell me, simple as swapping crossmembers with everything on it. 5 on 5 discs, and has either 10 or 12 bolt rear to match says will bolt right up. question is, will this kill the load/towing capacity on my truck? I do plan on using it for towing so it's important to me. I would appreciate your guys help.

also one other thing that's been bugging me. what kind of rears are put in these things? it looks like a ford 9" with a differential cover. long bolts going through the from front to back. pretty steep gears too, 4.88's it seems.