from what i have read the bore on the m70 is 84mm = 3.36"
and the stroke is 75mm = 3.0"
am i wrong? Reason is i see 304ci
when it is not

http://www.wallaceracing.com/cid.php

That calculator is simply using: Volume = http://mathcentral.uregina.ca/QQ/database/QQ.09.05/pi.gif r2 h...... You end up with around 319CI or 5.2 liter displacement if you plug in your imperial values. But if you start with using the metric measurements ie- mm, you will get 4.98Liter or 304.36CI. So its because of a rounding error in your Imperial units.

try 84mm = 3.3071", and 75mm = 2.9528", it should work out better.

Actually it is because:

84mm = 3.307"

75mm = 2.952"

use those inches and you will get 304.36 cu"

what is the actual formula?

There's nothing wrong with the formula, but if you enter rounded numbers you'll get an outcome that includes a rounding error.

1" = 25.4mm, not 25mm