Below are examples of the connectors.
5 pin Male mic side
5pin Male Metal Inline Microphone Mic Connector Jack 5e | eBay
6 pin Female radio side
Female 6 Pin Microphone Plug with Locking Ring 70006 | eBay
The Cobra pinout is:----- The Grant/Magnum 6pin pinout is:
cobra pin 1 mic audio----- Grant pin 1 mic audio
cobra pin 2 Mic shield----- Grant pin 2 RX
cobra pin 3 RX----- Grant pin 3TX
cobra pin 4 Ground----- Grant pin 4 up/down or no connection
cobra pin 5 TX------ Grant pin 5 ground
Grant pin 6 13.2v
Since your base mic has no up/down control, is a dynamic mic and needs no bias, buying connectors similar to the above from Workman or some other source, wire as follows
1 cobra to 1 Grant
2 cobra to 5 Grant
3 cobra to 2 Grant
4 cobra to 5 Grant
5 cobra to 3 Grant
The cobra hypothetically has a separate mic shield from radio ground which is NOT the chassis and for practical purposes can be connected together and probably is in most mics. Buying the in-line cobra male connector and through a piece of cable with 4 wires and a shield connect as above. If you can't solder, any shop should be able to do this. The connector on the grant is the same connector pinout as the Magnum 257, but doesn't use the up/down function of pin 4 or the 13.2V for the up/down and condenser mic bias. If someone is selling a 5 pin cobra to 6 pin Magnum pre-wired adapter, it should work, otherwise check with a shop to make one for you or find a local ham can do this for you.