K38 would be another suspect , but you want to jumper the NNS first.
The K38 circuit can be jumpered at its relay terminal 3 and 1 , but it is much easier/faster to just check for power at terminal 50 of NSS with the key turned to Start position.
12 V test lamp here indicates AT circuit [ K38 ] OK...
The power chain is Ign sw -to K38- to NSS - to starter sol.
To make it easy all around . if one checks the center wire on the firewall
as you have done , that is the starter sol end of the circuit . The other end is the NSS , ..so your jumper is the correct next step. .. but you just have the wrong jumper terminals and are feeding the starter circuit through the Back Lamp contacts of the sw.
That is why your starter keeps turning.
If no 12 v at 50 of NSS, than ign/sw or K38 are suspect....