Limiting input voltage for buck converter
Your main problem seems to be that you can't find DC-DC converters that accept more than +40V input.  There are in fact many such devices (either chips or modules).

Here is one example.
DC DC CONVERTER VIN = 5.5V to 50V, VOUT=5.0V, 28W 
https://www.pololu.com/product-info-merged/4091
You can buy it at Digikey for $20.
https://www.digikey.com/product-detail/en/pololu-corporation/4091/2183-4091-ND/11586858

Here is another example
DC DC CONVERTER VIN=9V to 53V, VOUT = 3.3V TO 16.5V, 8A, 128W
https://product.tdk.com/info/en/documents/spec/dc-dc-converter/i3a_spec.pdf?ref_disty=digikey
I3A4W008A033V-001-R
You can buy it at Digikey for $22.74
https://www.digikey.com/product-detail/en/tdk-lambda-americas-inc/I3A4W008A033V-001-R/285-2720-ND/7321113

If you want an DC-DC converter IC the LTC3895 can accept up to +150V DC input.

Additionally, Linear Tech/Analog Devices has a product line called a "Surge Stopper".

Finally you an use a 40V zener diode and power BJT to make the equivalent of a linear regulator that would effectively limit the input voltage if you choose to use a 40V converter.