I converted to manual brakes and I haven't looked back.
I used an 85ish s10 manual brake master cylinder, bolted it to the powermaster to firewall adapter.
I got a vacuum brake pedal assembly out of a g body malibu at the junkyard. I popped out the pin and reinstalled it in the upper hole on the pedal arm (presumably there for manual brakes). I had this welded in place.
While I was at the junkyard I grabbed a pushrod from an S10 and a couple of other cars of different lengths. You'll have to rip the crap out of a vacuum booster to get one.
The pedal assembly has nothing to hold the pushrod in place to prevent it from falling out of the manual MC. So I made a small bracket shaped like a U on its side and riveted it in place on the pedal assembly between the two pieces of metal on either side of the path where the rod would go. In this U bracket I drilled a hole just large enough for the rod to pass through. I cut the rod to length and radiused the end to fit into the master cylinder. I then made sure the rod did not touch the U bracket I made through its entire movement.
Since the brake light switch is more or less a stop for my pedal (it keeps the pushrod under a slight tension even when pedal is released so it doesn't fall out of the center of the MC) I had to get two nuts of appropriate size to hold the switch in place as opposed to the metal bracket it normally sits in.
That's about all there is to it. Connect your brake lines, bleed your brakes, and go. The only problem with this conversion is that the powermaster adapter plate doesn't quite hold the master cylinder level with the ground so your fluid levels will be off but they are within range.
The brake pedal feel is great, not too firm. Much better than a lot of old cars I have driven with manual brakes. It's not too much harder than vacuum really and there is an excellent pedal feel.