I'm planning to do a disc brake conversion on my 64 Elcamino. I had 79 model brakes on my other one, and the brakes were good. This time I want to use the aluminum quick take up master cyl. like the one on my 87 Monte Carlo SS. Better brake feel, less drag, and aluminum is cool, but I have found out that the booster is different. I don't think the Monte Carlo booster will work on the Elcamino. Is there a booster that has the larger opening that will bolt up to the Elcamino.

probably not.
and why would you purposely put the tiny 10.5" "metric" brakes on a car that will easily accept cheap 11" brake parts that are available from any parts store using brackets that are available from most resto places?
hell, for about $500, you can upgrade to 12" C4 Corvette brakes and use a manual master cylinder and stop better with the same brake pedal effort as a stock power brake setup. but, of course, most 15" wheels won't clear, so some 16" wheels will be needed. but late 80's 16" Trans Am GTA wheels are light, cheap, and look good on any car.

I thought about 11 inch brakes, but there are 2 very good reasons for useing the 10.5's. First of all I have them. I wont need to buy anything. Second, I had 10.5's on my other 64 elcamino with no problems. I ran it for 10 years, only replaced pads once, and when I someone pulled out in front of me I left 30 foot black marks in the road. It seems to me that 10.5 inch rotors are good enough. My Elcamino weighs 3405#. My 87 Monte Carlo ss weighs 3420#, and my 94 S10 Blazer weighs 3800#. All use 10.5 rotors. Why would I want to add the extra weight to use 11 inch rotors? I think safety is important, that's why I want to switch to disc brakes, but I don't want extra weight if it's not really going to do me any good.

it's gonna cost you more to properly put the G body brakes on your car than it would to to just get some 11" rotors, wheel bearings, calipers, and brackets and mount them on your drum brake rotors.
if you try to use the G body spindles, you are going to need different balljoints and tierods, and special upper control arms to crutch the geomtery a bit if you don't want an inch of shims on each upper control arm stud, which can push the cross shaft into the space the headers are trying to occupy.
and then, once it's all done, you will have mucho bumpsteer and a loss of turning radius.
of course, you can also get aftermarket spindles that use the stock tie rods and balljoints but use the metric rotors and calipers, but most of them are cheaply made drop spindles that screw up the geometry worse than G body spindles. and they alone will cost more than the 11" rotor setup.

Thanks for the info. The last time I did the swap I didn't check anything. maybe the geometry was screwed up. I knew about the ball joint problem, but the lower is the only one that has to be modified. I thought the tierod was the same, but it has been a long time. I think I will do some carefull measuring before I do anything. I would still like to use the quick take up master cyl. if there is any way.

After just a little measureing I found that the tierod mounting hole is much further away on the g body spindle. This would cause a loss in turning radius. That's enough for me to drop the whole idea and go with the 11 inch conversion.