The rb26 is strong enough internally. And bigger displacement isn't always better, especially in twin charged motors. The rb also has plenty of support, not to mention parts are cheaper anyway if you order from Japan.
I had Mcdonalds for breakfast, Wendys for lunch and then I came home and ate chocolate and drank pop all night. If there's a big enough pot I'll enter.
If it's the Nissan truck that has a ka in it any of those rb/sr motors mount up fine but the rb series you'd need a r30 series crossmember. I don't know if those will work in that chasis or not. Keep it Nissan, no matter what, ftw of course.
First screenie I could find:
http://media3.liveleak.com/xyz41/2006/Dec/29/LiveLeak-dot-com-20530-DSCN6788.jpg
I can't even tell what's going on in it, though.