some thoughts.

your original booster is a single diaphram booster. It does not produce enough boost for disc brakes.
It will stop, but you will not be able to lock up the wheels during a panic stop at all. not that built in "abs" is a bad thing, just pointed out you will not have full stopping power.

reusing your old spindles, means you want a conversion kit. not sure why you want to do this. for the same cost you can get disc spindles.
Regardless, if you have filled your wheel wells with tire, that is you are on the edge of your fender already with big front tires, be aware, that most of the conversion kits will push your wheel out.
most people don't notice unless they have large enough tires to see they now go past the fender.

the go to guy for this is dr diff on this board. he should be able to hook you up with what you need.