check out LC Engineerings tech nots section, they have lots of info on it.
"Using a 20R Head on an 85-95 22RE Block
--------------------------------------------------------------------------------
It is possible to run a 20R Head on an 85-95 laser block. In order to keep the cam and crank centerline consistent with an 85-95 head, you must mill the 20R head to a measurement of 3.130 (measured from the bottom of the front cam journal to the deck of the head). Remember to re-machine the oil passage back into the deck surface of the 20R head after milling to retain oil passage to the top of the motor."
thats for the 22re laser block, ive heard that even less has to be done to run it on a 80-84 pre-laser block.
why do we do this? to tell the truth im not completely sure myself. looking at the specs, its clear that the pre-laser 22r as well as the laser 22r has a better bore size (92MM - 3.622"), but the pre-laser block has a better deck height than the laser block ( 9.120" (Crankshaft Center to Top of Deck) on the pre laser) and ( 8.950" (Crankshaft Center to Top of Deck) on the laser block. these specs are better than the 20r block in most areas.
then, looking at the cylinder head specs (below) you see that the older 20r and 22r blocks have bigger chambers.
1975 1982 20R Cylinder Head
Round Intake Port / Round Exhaust Port
80 / 81cc Chamber
In. Valve Dia. 43mm / Ex. Valve Dia. 35mm
--------------------------------------------------------------------------------
1981 / 84 22R / RE Cylinder Head
Square Intake Port / Round Exhaust Port
82 /83 cc Chamber
In. Valve Dia. 44.5mm / Ex. Valve Dia. 36.5mm
--------------------------------------------------------------------------------
1985 / 95 22R / RE Cylinder Head
Square Intake Port / Pear Shaped Exhaust Port
58 cc Chamber or 22RET (Turbo)
In. Valve Dia. 44.5mm / Ex Valve Dia. 36.5mm
it looks like the idea is just to match up the best parts with each other for the optimum deck hight, chamber size, bore size, and im guessing head flow (rates not mentioned on flow but ive heard the 20r has better flow than the later modles).