I have a b21 kicking about which is known to be healthy, is it easily done to bore the b20 to accept b21 pistons? Are M rods usable in this application?

Easy, yes. Acceptable? B21/B23 rods are too heavy & so are the pistons. You are giving up a lot of acceleration if you choose to use them. If you are staying with the Volvo 44mm inlet valve, a 6-bolt bottom end which has lighter rods which are probably stronger in tension & .040" oversize Mahle pistons will make the same power & last longer + you don't have the nuisance of non stock head gaskets.

For a 2131 I use ACL Duralites for 186's, but my usage of Holden pistons was due to them being both incredibly good & cheap. (About a quarter of the price of stock style Mahle pistons in the late '80's)

There's lots of ways to build up a B20. If you are staying with Volvo 44mm inlet valves, then I would use 90.5mm as the bore for a race engine. My current thoughts for a good bottom end is to use Scat 4G63 rods, offset grind the crank to 89mm & get custom 90.5mm pistons to match. There is a place in Brisbane that does them for $150/cylinder (I recently heard). 2290cc.

If you are springing for a bigger inlet valve & paying for an extreme amount of porting, 92mm bore with 89mm stroke + the hassle of using expensive custom head gaskets. 2366cc, or 3.3% larger.
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Three 164's, Two 144's, One 142 & a partridge in a pear tree.