I am about to build a Kyosho Spitfire 40 that has retracts and requires 5 servos for controls plus a sixth for the retracts.

I have 5 x HS-325HB but need one for the retracts. Can I use another HS-325HB or do I need something with a bit more torque?

Yes you need more torque than what a 325 would put out. Also, you may need more travel than what a standard servo puts out as well. Retract servos normally rotate 180 degrees (and are non proportional).