Yeah, that picture uses metal screws (with the appropriate washers that you can't see), so the collector connection is on the other side, with a screw-lug under the nut. The connection is from the can, through one of the screws to the lug. Here is a crude example (though they seem to be missing their nuts):
If you use plastic screws, then that lug needs to be mounted on the other side, right against the tab of the can.
Notice the black washers that insulate the screws from the heatsink? That's what you need if you don't use plastic screws. It's stronger that way, but you would need to find those washers somewhere.
The MJE340 transistor, which should be mounted in the middle for the best heat-sensing, has an insulated case. So you don't need
an insulator, but you do need something to help conduct the heat. Heat-sink compound or one of those same T03 heat-conducting pads would work.