Inspection of figures 17.17 through 17.19 in the ARRL Handbook suggest that the components in your Pi network might need to be adjusted:
Jim Tonne, W4ENE, wrote a utility called PIEL to calculate output tank values. A Pi-L network - a Pi network with an L-network extension - is frequently used to match a wider range of impedances than a Pi network alone can handle.
To use the program, you must know the load represented by the plate of the tube. PIEL includes a "Plate load help" button to perform this calculation using your intended voltage swing, RMS output power and class of operation.
Assuming you want the plate to swing over half the 100V supply to produce 20W in class C, PIEL calculates the plate impedance to be 62.5$\Omega$. Entering this value in PIEL's main screen with an intermediate impedance of 50$\Omega$ to drop the L-section yields this Pi network:
Increasing the plate voltage swing and reducing the network Q reduces the capacitor values. You can experiment with these values until you find a network you can build with available components. For example, increasing the swing to 90V and reducing the Q to 3 produces this network:
But, you need to keep in mind that the network may no longer provide the harmonic rejection required to comply with FCC rules.