I see that front struts are being made but my factory manual talks about removing the rod and filling the tube, packing the top seal. So does the new strut fit inside the existing tube?

Yes

What holds the new strut in? Looks like there is something on the bottom of the new strut?

Perhaps it would help a bit to clarify some terminology. The strut is the upright tube and its assembled parts. The shock is a hydraulic dampener that fits inside the strut tube. The OE shocks were an open design (no shell cover) and used the strut tube as a reservoir to contain the hydraulic fluid. That is why your FSM refers to filling the strut tube with hydraulic fluid. New shocks are sealed units that simply slide into the strut tube. They are secured with a large nut called a gland nut, the same as the OE shocks. Most new shocks suggest that a small amount of oil or hydraulic fluid be addded to the strut tube to assist with heat transfer. Most new shocks also provide a gland nut that is designed to match the shock. Hope this helps a bit.

Not sure who actually recommends adding oil, but KYB specifically says don't, and it looks like they are the only readily available choice.  I assume that Koni is still supplying struts too, just through the Z Store.  Technically, it is still a "strut" too, which can help part searching.

https://www.rockauto.com/en/catalog/nissan,1978,280z,2.8l+l6,1209260,suspension,strut,7584

https://www.thezstore.com/page/TZS/PROD/23-1033

 

Oil or no oil is a hot topic -

 

I use the term strut insert. If you have ever fought removing a rusted strut insert from the housing you will reconsider adding a small amount of oil/anti seize just to prevent rusting. 

I dont recall early z's using anything but an insert. Did they have a true hydraulic strut that used the housing as part of sealing from the factory? I know my 87 911 was made that way, as it was a big mess when I pulled them out. I wasnt expecting all that fluid!

7 minutes ago, emccallum said:

I use the term strut insert. If you have ever fought removing a rusted strut insert from the housing you will reconsider adding a small amount of oil/anti seize just to prevent rusting. 

I dont recall early z's using anything but an insert. Did they have a true hydraulic strut that used the housing as part of sealing from the factory? I know my 87 911 was made that way, as it was a big mess when I pulled them out. I wasnt expecting all that fluid!

All of the S30 series cars used the strut tube - front and rear, as the reservoir for the hydraulic fluid. The shock or insert as you term it was an open design with no protective casing or shell. The FSM covers it very thoroughly. Today's shocks are fully encased/sealed units. +1 on your comment of adding a small amount of oil to the tube to prevent rustwelding of the new shock to the bottom of the strut tube. KONI recommends it, apparantly KYB does not.

4 minutes ago, jfa.series1 said:

All of the S30 series cars used the strut tube - front and rear, as the reservoir for the hydraulic fluid. The shock or insert as you term it was an open design with no protective casing or shell. The FSM covers it very thoroughly. Today's shocks are fully encased/sealed units. +1 on your comment of adding a small amount of oil to the tube to prevent rustwelding of the new shock to the bottom of the strut tube. KONI recommends it, apparantly KYB does not.

 

Thanks, I guess I was too young to remember that when watching my Dad/older brother work on z suspensions. By the time I got old enough the originals had already been replaced! 

How do i get the top nut/cap off the strut? I tried heat...nothing. the nut is sheey metal and trying to bend

A big pipe wrench and lots of heat.  Put it back on the control arm to get more leverage, use a long 5/8" bolt where the spindle pin would go to make it easy.  Don't crush the tube.

Air chisel has worked for me. Something about the vibration helps loosen them up. Of course heat and kroil oil will help too. I wouldn't worry as much about that nut as I would about the insert being rusted in the tube! 

Kano Kroil oil let it sit overnight