The oil gauge will need a separate pigtail for the sender. This is for the 305. The oil light pigtail is shared the electric choke (3 wires, 31 tan-oil, 78 light blue-choke, 250 brown-choke power from fuse.) For the gauge converson, the 31 tan wire will need the separate pigtail, the 78 light blue & 250 brown will need another circuit for the choke light (oil warning light) which is 931 dark green/white stripe. You could go into the dash & swap wires in the cluster connector by moving the 31 tan (located between the 25 brown & 35 double green) to the open spot between the 33 double tan & 237 yellow. Then run a wire from the gauge sender to the spot the 31 tan was. This way you can still use light sender with out cutting wires. If you cut the 31 tan, splice the gauge sender pigtail, the splice the the wire for the 931 dark green/white to the tan wire on the pigtail (leave enough wire to work with) & run it to the open spot between the 25 brown & 35 double green. If not using the choke light, use a good crip cap to protect the unused end.
www.maliburacing.com is a good site that should have a link to wire diagrams to give an idea what I posted (hope you stayed awake reading it all.) Also, if you got the cluster connector, it will have the ends need to show what goes whwere & wire ends you'll need.