It will fit fine you will need to do some drivetrain upgrades to handle the power.
No need for the longer chassis, it is just a matter of preference.
On the downside I have heard a lot of quality issues with the 5.9. It is made by Axial for HPI.
Axial is known for using poor quality bearings and leaky carbs.
I suggest you find a LRP Z28 spec 3 or an SH Pro 6 port .28. Both have tons of power, Both of these are much cheaper than the 5.9 and are fast as hell, very reliable too. The SH comes with a roto start, the Z28 has to be converted to roto.
You'll need to switch to a 16 or 17T CB to gear these engines properly.