There is also a book called "Welsh Words" (in both a north and south edition) which seems to be based on research, more recent than 1967 I think. I don't have the book though, and I don't know how many words it has.
I won't give a link (it's easy enough to find), but here is some stuff copied from the description:
Welsh Words - Geirfa Graidd, Lefel Mynediad (De Cymru/South Wales) Paperback – 20 Oct 2014
by Steve Morris (Author), Paul Meara (Author)
This is not a list of every Welsh Word you will ever need, nor a long list of words to enable you to cope with the Mynediad level course for adults. This is a core vocabulary and is based on research by experts in the Welsh for Adults field. These are the words you will need to know by the end of the Mynediad course level. The easy to use size means it will fit neatly in your bag. Nid rhestr hirfaith yw hon o bob gair Cymraeg fydd ei angen arnoch i ymdopi gyda Chymraeg lefel Mynediad i oedolion. Rhestr o eirfa graidd yw hon ac mae'n seiliedig ar ymchwil gan arbenigwyr ym maes geirfa/Cymraeg i Oedolion. Dyma'r geiriau fydd angen i chi eu gwybod erbyn diwedd y cwrs lefel Mynediad. Bydd y maint hwylus yn golygu y bydd yn ffitio'n dwt mewn bag.
In passing, I noticed another book (unrelated I think, and earlier) called "6,000 Welsh Words". I don't know if that is based on any research. It gets good online reviews, FWIW.
That kind of book looks useful, but on the other hand, I'm not quite sure how one would actually use it in practice.
Similar to my thoughts.