Hey there,
thats awful news, was hoping for some nice acceleration grins. Anyway, wish you even more luck for the new block. Concerning the pistons out of the B21, as far as I know those have 24mm conrod pins. They are quite popular for bored B20's but you need the right conrods. Tuners I heard of shrink fit the piston pin into the conrod. Supposed to be the best, most reliable fitting soultion.
Make sure you don't mix imperial stuff with metric stuff. The change to metric engines was in '73 if I'm not completely wrong.
Have a look here if you are in need of mint new conrods...
http://www.pauter.com/exotica.htm
Good luck,
Bruno