I'd like to swap over the front power disc brake setup from a '72 elky donor car to my '68. I know I'll need to trade the proportioning valve, but will my existing drum power brake booster/master cylinder handle the discs? Are there any other considerations I should keep in mind? Thanks!

I think you will need the master cylinder from the donor car also.

Yes you definitely need a master cylinder from a disc/drum vehicle. The disc calipers use a much greater volume of fluid than the wheel cylinders. You will need new rubber brake lines from the hard line to the caliper. Hoses from a 69 camaro disc vehicle are suppose to work well.