High Voltage Buck converter 65VDC --> 42VDC

[jeanlion1]

I have several 65VDC 300mA  powersupplies  (230V --65VDC)  ... I would like to design a buck converter which allow me to regulate the fixed 65VDC output to an adjustable 40 --55VDC output. Is there someone who can guide me thru the design process? Is there an online tool like "TI's webbench power designer" to perform this task properly?  I have studied a  bit the basics of the buck converter technology, however I lack experience in design.
Also I'd like to know which testing gear I'd need to evaluate prototype designs.  Then remains question which reverse feedback (reflection) occurs into the existing powersupplies.   A device which seems appropritae may be the "STMicroelectronics L7983 Synchronous Step-Down Switching Regulator", however I am open to any other component ideas.
Thanks for your ideas and comments.

[Terry Bites]

TI have an online design tool that's right up your street. https://webench.ti.com/power-designer/switching-regulator/select
You have to create an account to use it. I put you paramters in and got 35+ potential solutions.