that's mad... you're avatar with knee down doesn't help me thinking that you must have been giving the car abit of stick from time to time The engine shouldn't blow though! Don't think petrol is the cause but sure what do I know. Do you fill the car up at the same place? What oil are you using, and do you change oil filter when you swap the oil out? I've never heard of engines blowing like described for Z's until now! I'll keep an eye out for an engine, but if you can't source one via Mark or R35LEE, well...
Is there any chance of your old engine and your brothers old one able to be repaired? (like bits from both engines plus new parts)
Can I also ask how you drive it? (I've heard these don't like to be reved like an inline four )