Get rid of the Rb20 and use an RB30 bottom end with a 25 head.
Much more possible to do and a RB30 botom end costs about $100 with pistons, rods, crank even water pump and balancer.
All they usually need is main/big end bearing replacements, new water pump, tensioner bearings and a few other basics like a quick crank shave and hone and you can have a RB3025 which is possible for one thing and you get alot more out of it.
IE the obvious 1ltr increase.