Aye, I should have been clearer, I didn't mean for the original decision, nor for now, but in case the HSTs aren't ready by the 2020 deadline.
It could be possible, there's different considerations if you're advocating the idea as a stop gap until the full refurbished HST fleet is available, or as a wholesale replacement for the Scotrail HST programme.
If using 67s and mark 4s as a stop gap until all the HSTs have been refurbished, it should be noted that the mark 4s aren't available until after the 2020 deadline either, it would require additional crew training (and possibly route clearance, mark 4s haven't been further north than Inverkeithing/Rosyth Dockyard as far as I'm aware) on the type, and time and cost to convert the DVTs to AAR multiple working, when a derogation to use "classic" HSTs for a period of time might be more straightforward.
If as a wholesale replacement, then it would be a struggle to source sufficient locos if Transport for Wales intend to continue using them with their three planned push-pull mark 4 formations, even assuming DB Cargo have no other work for the class following the cessation of the contract to provide 'Thunderbird' locos on the East Coast post-IET introduction; and also a struggle to source sufficient stock if LNER retain any sets as stated in the franchise agreement, or Alliance Rail are still chasing after any as has been rumoured for their proposed West Coast operation. There are also the limitations of class 67 locos in terms of acceleration and inability to take advantage of differential speed limits that may render them incapable of delivering the journey time improvements proposed with the Scotrail HST fleet.
None of this necessarily insurmountable; Transport for Wales will be doing exactly this albeit on a very small scale in what I would consider a similar operating environment; but a few points to be considered and either way it'd also be a fair few contracts and agreements to arrange or renegotiate at short notice.