Yes, there are quite a few variations. The more literary versions (used by southerners too) are:

gennyf

gennyt

ganddo

ganddi

gennym

gennych

ganddynt

And you'll get all sorts of variations in between those and the more colloquial ones I posted.

I'm interested you had family from Denbigh as I had Welsh speaking family in roughly that area too. The dialects in the north-east can be quite distinct from the more often-heard north-western ones which people usually think of as "northern Welsh".

The only book I know that lists all sorts of colloquial variations together is the massive Welsh Dialect Survey. I remember spending hours poring through its lists and lists of words in the library, but you can get a secondhand copy yourself off Amazon if you have a spare £2000 ;)

There is also Cymraeg, Cymrâg, Cymrêg, which goes through different speakers they interviewed area by area pointing out accent, grammatical and vocabulary features. It's all in Welsh though.

If you're interested in the traditional variety of Welsh in Denbigh and the surrounding area, Welsh publishers sometimes produce little books introducing dialects, usually full of lots of interesting words. I can't find a north-eastern one at the moment but I was thinking things like the Llyfrau Llafar Gwlad series by Carreg Gwalch. In the meantime there are few lists (in Welsh) on these archived BBC pages: 1 2 with people's comments at the bottom too. I still use the word sbensh even though I'm a southerner!