Thanks for the extra information on the 77. I know that at one time HW did use 'square section' springs for a while but was not aware they were fitted to the early 77 like yours. I have a 'MK2' 77k that came with a round section spring.
With regard to price, my feeling is that, sadly, your 77 will not fetch as much as an unaltered one. Maximum of £120 in its current state and possibly less if you take into account the need for a new underlever ... can you get that model u/l now? I note Chambers have the MK3 u/l new at about £83 but do not know if this would fit your model. Maybe a search of various auctions etc would find one eventually. Of course if someone is happy to accept the modifications then you might get a higher price.
Sorry to let you down like this but these thoughts reflect my own reasoning and, of course, many may not agree with me.
Cheers, Phil