Tom,
i'm not sure your statement is correct that the canadian manifold is "bigger" than the 313. We ran back to back tests on a street stocker (circle track car)
and there was no difference in power. So, we sold it to the competition. If your running a class car that requires the 313 the you need to get ahold of guy by the name of brzezinski at
www.cast heads.com.
If your not restricted to the 313 but has to be cast iron then start looking for the following, and i'll tell you now there hard to find but worth it. The casting numbers are as follows 14088674 or 14088675. They originally came on bus's/trucks in the late 60's early seventys. The quick visable difference is that the carb. Pad (plenum area) is 3/4 inch (lil mini tunnel ram). Taller than the 313.
Danny, haven't talked to you in a while, hope everything is well.....
Norm