Just thinking

So one block is 3.3 ft (1m)
The smallest (non modded) possible fuselage is (0.25m×0.25m×0.25m) or 1/64 m^3 (0.015625m^3). Alternatively (3.3ft×3.3ft×3.3ft) is 35.937ft^3. Weighing in at 9 lbs (~4.07 kg).
You can fit 1000 lbs (453 kg) dead weight into it and no fuel
Thus its density (metric) is 457kg/0.015625m^3 or *29248kg/m^3.
(Or in imperial its 1009lbs/35.937ft^3 or ~28lbs/ft^3)

NOW THE REAL STUFF

We will convert it to g/cm^3 for comparison to osmium the densest naturally occuring material at 22.59g/cm^3
We will restart the math
The fuselage is 25cm×25cm×25cm or 15625cm^3 and weighs 457kg or 457000g
457000g/15625cm^3 is 29.248g/cm^3 which is 6.658g/cm^3 denser than osmium.

So i dont know what a fuselage block is made off