Plastering Internal Doorway - Looking for Advice

I've removed a door and lining from an internal doorway and seeking advice on how to plaster the inside of the doorway, and how to tie it in with the exisitng plaster. I'm planning on keeping this open, and no door will be hung.

I've attached a few pictures showing the state of the existing plaster and blockwork, as well as the steel lintel which needs to be taken into consideration. I wasn't sure if I should use plaster board or stick with hardwall as this appears to be what was used, and how to tie in the corner bead with the existing plaster.

Any help would be appreciated.

 

Thanks for the advice. Will I need to use scrim tape in the top inner corners to stop the plaster from cracking?