I have the 71 240z with series 1 seats, an adjustment knob...  Seats were out when i bought car. I am now ready to get them back in. I have seat in with front nuts on. The back nuts are the issue. Taking into account the carpet its virtually impossible to even get one hand underneath to tighten nuts much less two hands.

Anybody else have this issue? what did you do? I am contemplating taking off the back rest to see if that will help me a little bit.

The carpet section and jute pad (if still there) should easily be removable from under the seat. Since you have the front nuts installed, rake the seat back forward and then slide the seat fully forward. That should get you enough room to install the rear nuts. Yeah, its snug but doable. I've had seats out and back in any number of time, never had to remove the seat back.

Slide the seat all the way forward, recline the set forward as well...this will give you your best access.  It is challenging no doubt.  Slide your fingers under and feel around to get the nut started, you will be doing this blind as there's no way to be able to see what you are doing under there unfortunately.  Once you get it finger tight use a combination wrench to tightening it down.  This is about the best description I can think of right now.  Hope this helps.

I just did this recently.  Use some plumbers putty to stick the washer and lock washer on so you don't have to hold them. You can then get the nut started with one hand. 

14 hours ago, Vorticity said:

Use some plumbers putty to stick the washer and lock washer on so you don't have to hold them.

I use white lithium grease and a fine tooth racheting box end wrench. (Gearwrench:    https://www.ebay.com/itm/GearWrench-KDTools-Metric-7-Piece-Flex-Combination-Ratchet-Wrench-Set-9900-NEW/300930317149?epid=1717817801&hash=item4610d83b5d:g:yp8AAOxyAc1SPMfm ) This method has saved most of my cuss word vocabulary for other issues.

I forgot to mention the ratcheting box end wrench.  I've got a different model that I used, probably from Harbor Freight.  

As i'm tall i used 4 (2x2) from those big plastix nuts there are used under the seat, to lift the front up.. if you then put the seat in the front position it's also easier to do the rear nuts. i used a flat ratcheting wrench.

i got them in...sigh. but now need them back out... what a pain. 

part of the issue is the bolts are not that great. Has anybody successfully removed the bolts and replaced with new ones with really good threads? Something is keeping the bolts in. My eyes are not great and i cant tell it they are welded in or something else

I used a die from my tap and die set and carefully cleaned my threads. I can now break them loose and turn with fingers. Its hard to put seats in their position without scraping threads a little on my car and who knows who removed them in the past and didn't use care but mine were a pain to remove even with ratchet wrench. Now its not bad at all to remove seats now and we all know how much easier things are to work on without them in car.