You might not be able to fit 5 26650s in a 5D maglite. I know that 3 will fit in a 3D maglite with a very small spring and/or tailcap modifications, but 5 might be very close.
Also, are you planning on doing something over 180 watts? If you want a hotwire with that level of output, you will need to replace more than just the lens and reflector; the switch will have to go as well so it doesn't melt. Since your maglite is over 20 years old, the current metal maglite switch/driver assemblies night not fit properly. The Batteryspace cells are the best value in my opinion, and I would think that a 10 amp draw on 5 cells would result in a light that satisfied your needs...
However, the batteryspace cells are rated for up to 20A for short bursts, so they would probably be ok with a 13 amp load.