CAD geek here.
You will need atleast some kind of margin between the two surfaces, or you will have a fit that is too tight, and while you can push it in probably, there is a chance (depending on printing filament) your piece will start to crack at the surface joint.
For as to how much margin to use, look up the minimum tolerance of your 3D printer, and use that as a rule of thumb.
If I can give you another tip, I would suggest to remake your joint with primitive shapes, this will make it much stronger, it will be easier to adjust your margin, make positioning easier, and keep it in place much better.
Example made in 2 mins with Booleans below :