Hi guys,
Many thanks for your replies. I have been basing my assumptions that a secondary stamping of 3 is a 98cc head from a 400 4bbl MT on the following:
http://pontiacpower.org/4xheads.htm
Both heads are most definately stamped "3", both H182 casting date same as the 455 block. The motor has been apart before so I can't be sure the combination is original, but it seems ulikely a previous owner found a set of 400 heads cast on the very same day as the block.
It would therefore seem Formulabruce's observations may more likely translate and these "3" heads have larger chambers.
My next steps will be to check the compression and check for press or screw in rocker arm studs before ultimately removing to cc a head.
Thanks again for your advice