What to convert 69 chevelle fr drum to disk what year a-bodies will work
from salvage yard and can i keep the manual brakes or do i have to go to power brakes ?

You may want to look here:

http://www.chevelles.com/techref/ftecref3.html

"There was a limited number of factory disk brake set-ups available around 67/68 that used a dual piston caliper. It's my understanding that they are difficult to obtain and that parts are hard to find.

In 69 the single piston set-up was introduced and was manufactured through 72. The spindle, steering knuckle, dust shield, rotor, and caliper can be obtained from any GM A body car. It would also be possible to obtain the parts from a Nova (X body) or Camaro, however, the steering knuckle is different due to being "rear steer" and would have to be replaced."

There are later model options as well, the aftermarket, etc...

I just snagged from the spindles out from a 71 El Camino at the Pick-n-pull as well as the small brackets for the frame side of the flex lines. Not sure if it was a good idea or deal, but what the hell, they were there and I was too...

I think the master cylinder bore/piston diameter is different for disk and drum as well as the proportioning valve. Drum may not have one, so get it if you run into a setup at the scrap yard...