First off, kick ass find.
I would personally use the ground on the left side near the phono inputs, since you've mentioned the ground wire is in the same sleeve.
You can just snip off the ends off of something like this which came with my Sansui T60 Tuner.
Sounds remnant, though if I were to guess they were suggesting an earth ground or possibly an isolated ground, seeing as there was a potential back then that the actual two prong receptacle box that were common back then might not have been grounded at all.
This little guy should do it.
You can buy this for FM or look online on how to make one using speaker wire.
there appear to be black screw terminals to the right of the screw terminals for the speakers. that's where you'd connect antennas. Two are for the AM, two for FM. Use a simple dipole that used to come with most stereo receivers, like this one. The spade lugs can be screwed right to it.
I bought an antenna like this one. The spade terminals fit onto the posts and then you tighten the nut to hold it in place. Ancable 300 Ohm FM Antenna T Shape Dipole Radio Ant with Spade Terminal for Yamaha Sharp JVC Marantz Vintage Stereo Tuner Receiver https://www.amazon.com/dp/B07BGY21GZ/ref=cm_sw_r_cp_api_fabt1_Ci9VFbXCM8CX8?_encoding=UTF8&psc=1
agreed - like an old school version of this: https://www.amazon.com/dp/B07BGY21GZ/ref=cm_sw_em_r_mt_dp_RHW1SAC5SDXAEWDJHZ28?_encoding=UTF8&psc=1
You will be looking for a 300 ohm antenna
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>300 ohm fm antenna
this. Easiest solution. These simple things perform surprisingly well.
Is this the kind of antenna you are using?
https://www.amazon.com/Ancable-Antenna-Terminal-Marantz-Receiver/dp/B07BGY21GZ/
The reason I ask is it's not what I would consider "short," but it is the classic "basic" FM antenna. You have to hang it up in its T shape near your receiver (see product photos).