I did a bit of research about this awhile ago but I can't seem to find the solution. I have an M77 with an M76 buttstock attached to the slantback M77 receiver.  Fits fine but is a bit unsightly. I read that somebody makes a slant/straight adapter. Was this Ace? Krebs? Can I remount the M76 but stock using such an adapter so that all is in alignment and tight?

Also, what is the trick to getting the M70 wooden handguards to fit better on the M77.

The only adapters I am aware of that fill that gap are for bolt on folding mechanisms and stocks, not wood M76 stocks.
M70 hand guards should fit fine. What is the issue you are having?