I have been meaning to ask you
To just say this I think you would say
Quería preguntarte... "I was wanting to ask you (whatever you were wanting to ask)
Maybe in written Spanish you could say
He estado queriendo preguntarte... "I have been wanting to ask you..."
but that would be formal and stuffy.
The verb 'mean' is one of those verbs that cannot be used with continuous tenses.
So you can't say 'I'm meaning to say', this is wrong.
Your sentence "I have been meaning to ask you" is in the present perfect continuous, so it's wrong.
Other verbs in this class are hate, like, love, need, prefer, want, wish, believe, imagine, know...etc, sometimes they are used in continuous tenses in slang though.
For the Spanish part I'd simply say ' (yo) quería (lo) preguntarte' .
"Había pensado en preguntarte... etc." This is correct, but I'm not sure if it's the best way to say it.
You could also say "he estado pensando en preguntarte." Same meaning, different wording.
The way I would say it is:
He estado por preguntarte
Estar por means "to be about to" or "to be on the verge of". So this would be like "I've been on the verge of asking you".
The dictionary says:
to mean (intend): "tener (la) intención de hacer algo"
"Llevo teniendo la intención de preguntarte..."