Bottom bracket threading question.
 

I have a 1977 Puch Marco Polo that came with Shimano Selecta bottom bracket and cranks that I would like to replace/upgrade. I removed everything and now need to buy a new bottom bracket since selecta can't work with square taper cranks, but can't determine the threading I need. Is this bike just standard English/ISO threading.

Any help would be appreciated. Thanks.

ISO threading is very likely -- see if there are any 1.37x24 markings on the cups. If you have a known ISO lock ring, see if it spins on freely. If it binds, you probably have French or Swiss threading. For what it's worth, both of my Austrian bikes are ISO-threaded, but I have heard unconfirmed reports that Steyr-Daimler-Puch sometimes used French threading, so there was evidently not a national standard.

So today when I tried to put an ISO threaded cartridge bottom bracket in, the left cup would go in turning it towards the back rim (right). However, the right cup and cartridge would not screw in....

AFAIK Selecta only came in ISO threading, so the bike should be a "standard" 68mm shell with a left hand-thread on the drive side and a right-hand thread on the non-drive side. You state that you got the left-hand side retainer of a cartridge in, which would confirm this.
If you're having trouble getting the cartridge to go in, make sure you clean (and really clean) the threads well. If there wasn't much grease on the threads to begin with, then some corrosion may have built up, which definitely makes a new cup hard to get in. A good idea would be to grease the existing Selecta "cup" and screw it all the way in and out a few times to chase the threads. If not, get hold of a dedicated bottom bracket die.