One more thing, after going to Mummert's site, research block ID and head ID. Then check you engine to see what is there.
The late truck 292s came with a 2V Autolite carburetor, with small 1.02 venturi. Flow is 248 cfm I think. You sure wouldnt want to go smaller than that.

Here is a marine 292/312 intake with side draft carbs currently on ebay: http://cgi.ebay.com/ebaymotors/Y-Blo...QQcmdZViewItem