Has anybody ever tried to use a drum spindle for disc brakes? It seems that I will have to cut that upper mounting lug down and possibly thread the hole a bit deeper but it looks doable.

I remember seeing kits that retain the drum spindle set-up.
Can't remember where or how long ago.

The drum spindle kits may be only for the old cars that didn’t ever have the disc option. Suck it up Bob and replace the spindles

C'mon Bob, where's your sense of doing what they say can't be done? 😉
A new set of spindles would definitely be easier and if you think about time involved probably cheaper in the long run. Then there's the whole thing about the integrity of a 50+ year old part compared to a brand new casting. But I'm hardheaded that way. I may eventually end up ordering new but I'm just curious enough to see if I can actually make it work. What's the worst that can happen? I can ruin a good spindle and possibly cut one of my fingers off trying. When did that ever stop me from doing anything?
YouTube video to come. 😎

Here's a right stuff kit from summit that uses the drum spindles .
There is also a kit with out the booster that is $200 cheaper

https://www.summitracing.com/parts/rsd-afxdc14/year/1968/make/pontiac/model/firebird

This kit is for manual front disc brakes but it comes with the spindles
https://www.summitracing.com/parts/rsd-afxsd02c/year/1968/make/pontiac/model/firebird

Well the thing is I already have a set of orig front discs. Everything but the spindles and hoses so I’m not buying a kit just to get spindles. Actually I got out there and got after it today. Got everything mocked up. Not as difficult as I thought it would be and came together nicely.

Well that's cool , I ordered the kit for the manual front discs it turned out good but there was a little bit of work involved because
you have to use a different master cyl and prop valve for front disc and you have to change the hole that you use on the brake pedal
for a different throw for the new master cyl for discs
I'm just telling you this because you can't use the old master cyl or prop valve to make everything work correctly .
i guess those dam engineers knew what they were doing well some what lol

You must’ve gone from std to power brakes. I think that’s the reason why you have to change rod position on the pedal. You can use the same distribution block but there are a couple additional valves that were added to a stock disc/drum arrangement. Not to worry. I’ll configure it to work. Thanks for the heads up tho.

I went from std drum to std disc I think the difference is the std drum master cyl has a 1" bore and the disc master
cyl has a 1.25" bore for more volume for the front disc calipers

Makes sense I guess. You are going to need more line pressure for disc than for drums either boosted or not.

I have manual disk/drum brakes on my car. The piston bore is 3/4" or 7/8"
Power brakes use a larger piston bore.

Mine was changed from from drums to discs. Spindles are drum, don't know if they were turned but I don't think the F cars needed to be. I still have the drum drum master cylinder, just had to take out the residual pressure valves in the master for the front brakes. One inch pistons.

No the issue with the drum spindle is the top bolt hole that the shoes use as a kind of anchor bolt. The lug on the spindle needs to be cut off about .600 so the caliper bracket can bolt onto it. There's a couple other things that are different too but basically they just require changing to different size bolts and a bit of shimming.