I'd give it a go with the 5.3 feet that you have, as we tend to supply more wire than is needed both in the bill of materials for a given kit, and in the packing process.
The wire is 20 AWG solid core PVC insulated hookup wire. We do not recommend using stranded.
If you want to bypass the headphone jack, you can wire the twised pair of wires from the output transformers (terminals 7/8 for 8 Ohm speakers) right to the binding posts. Just be sure you use the twisted pair that come off the secondaries, not the primary (the primary feeds each octal socket).
If you change your mind about the headphone jack later, you can wire from the speaker posts to the headphone jack, which would only mean that you wouldn't get the automatic muting of the speakers when you plug your headphones in.
Out of principle, I would wire it as-directed in the manual first.
-PB