If I was you I'll stick to the B7 and swap the head only. I have a hybrid and I didn't regret losing the Z6 block. I'll let you know why:
The B7 with the Z6 head will increase the compression, pistons are high, Z6 pistons are low. Because I'm NA, that gives me more torque on the wheels. I used the P28 ECU and ran only two wires: VTEC solenoid and Oil pressure, don't needed to re-wire the whole car. If you do the swap, the cost of it will be no more than $300 in gaskets, timing belt, sparks, water pump and other stuff you will need.

Also another reason to stay with your block is that you know what you got under the hood. The other engine, is in good shape? If you need more info regarding the swap I'll more than glad to help you...