Howdy all I recently bought a 68 400/4speed frame off resto in Nightshade Green.
I plan on upgrading a few things and many questions to follow:
Currently has 4 wheel manual drums. Would like to at least add disk to front.
Rear suspension seems very heavy. Will have questions on that and what are recommended fixes.
Upgrading steering box to quicker ratio and less assist.
Looking forward to everybody’s thoughts and ideas.

"Rear suspension seems very heavy"
What exactly do you mean by this?

It rides like there’s 4 people hiding in the trunk, like the old drive in days. Bottoms out a lot. Thinking about stiffer springs.

Stiffer springs will make the car ride like a 1 ton truck. Don't do it.
Anything over a spring rate of 180 lbs and you will truly will rattle the teeth out of you mouth when you hit every crack in the road.
Inspect the entire rear suspension. Bushings, shackles...etc.
My guess is the springs are old and warn out.

WOWSERS that is clean! Excuse me while I roll my tongue back up off the floor.

If truly a 'frame off' I would imagine they used new rear springs. Something does not compute. Maybe they cheaped out and left the old springs there? Maybe the shocks are shot? Does it bounce around? I agree with Bigchief- don't go crazy with HD springs. You want compliance with the road. How does the front feel? I had front discs in my 1968 hard top. WAY better than drums. Now I have 4-wheel disc and that is what you want. Lots of aftermarket kits to make that happen.

Everything looks new underneath. The springs very well could be restored but worn out. Shocks look like NOS but could be worn out too. Probably start replacing them first then springs.
Front end feels fine. Originally was going to replace the 4 wheel manual drum with 4 wheel power discs but not sure all that's necessary. Ive heard simply adding discs up front keeping, not adding power booster, would be a massive improvement.

It does look great. The problem with NOS is that they are still over 50 years old. Seals get hard, leak, etc. I could care less about originality, but, there are plenty of aftermarket reproduction shocks that look exactly like the originals, but are modern versions with much better valving inside.

I used to trailer a small boat with my 1968 hard top. I used 2nd gen leaf springs as those cars were heavier. They managed the boat without being too stiff for normal driving.

Have you done the usual tests? Push the rear down and see if it rebounds and stops right away? Load the trunk [on a blanket] with cement blocks and see if it sags? Did the restoration include receipts?

Good looking '68.

My '68's factory original springs only lasted until 1976. At that time it was bottoming out often. That GM replacement set lasted until 1987, when I did the second replacement. A peculiarity is that the factory original springs for a 400 standard suspension was a five-leaf spring and a heavy duty option was a four-leaf. Note that the car never rode great but it isn't a Bonneville. As mentioned, shocks can make a big difference, I did three or four rear shock replacements over the years. Currently I have a set of those "correct look" spirals repros on mine now for an originality look. As mentioned, a more modern robust shock may offer a better ride.

Be sure you have the two axle bumpers (within the wheel wells) and pinion bumper (above the pumpkin). They won't affect the ride but limit harshness and perhaps damage when bottoming out.

I don't see the exhaust system in that photo.
Edit: never mind, it looks like a loose tailpipe on the left side. Maybe it's being installed while on the lift.

Front disk Yes
Rear disk is a waste of time and money. 70% of stopping is on front brakes and the aftermarket kits for the rear is garbage.
Disk / drum will give you the best stopping power and the best pedal.
For manual disk/drum use the corvette master cylinder for manual brake car.

I still prefer 4 wheel discs. I also tend to change to 4 wheel discs whenever possible just to avoid the torture of servicing drums. I can't think of a single vehicle that still has drum brakes. But what is worse is the 4 wheel disc set-ups that use a small drum brake inside the rear hat for a parking brake. Hard to service and a piss poor parking brake too. In that regard the second gen Firebird rear discs are the best to use.