I'm building an engine with Chris soon and he doesn't like RB26/30s in GTRs. He may build an engine alone for you if that's all you want and you're adamant about going down that path, I think it's mostly the installation in GTR process he's not a fan of, but will prolly talk you into a stroker because he reckons they run smoother and produce much the same power as a dirty anyway.
And yeah call any workshop about your plans before you buy your bits...some workshops, my former self included, don't like it when people just rock up with a mixture of self purchased parts and then expect it to gel together and some sort of warranty on top of that. A reputable engine builder will have played around with different parts and found what works best for them and what they build; I'd be going with stuff the builder recommends, even if you source it yourself. RacePace are pretty good on parts there's not much margin for it - worth it for me when you can leave it all in their hands and it's another build that they've done a hundred times before.