Hey mate,
Just Go the Maddat stuff.
No mucking around with measurements, guessing etc.
An engineer will see that it's come from a reputable company (if thats your plan).
All too easy!
My advise anyway...
Maddat Website -
http://www.maddat.com.au/Crossmember - (small image sorry)
http://datsun1200.com/modules/myalbum/photo.php?lid=18304and No, I don't work for them. I Just like their products.