Talk to Baz from Datsport
Go for L20b straight off or Z22 block with L20b crank or
if its for engineering reasons you will need line boring
of the L20B crank and to fit longer L20B rods you will
need block line bored as mentioned by Mick above which is
about 200-250 dollars at best but then you use L16 rods,
1mm oversize RB30E pistons, rod stroke ratio is 1.547 which
means it will be a torque monster but keep it to 7500rpm
for max with good springs on the head along with closed
chamber head with decent porting to make up for faster
air going into the engine as opposed to long rod combo.

Try the calc w/ above details RB30 slugs 1mm over = 87mm
http://www.ozdat.com/ozdatonline/enginedesign/