Differences between "R" and Avenir/BB blocks?

[jerryeads at joimail.com]

Went searching through the archives looking for Avenir &
swap information.  Several members stated that there is zip
difference in the internals between our stock "R" sr20de
block/head (other than compression=pistons?) and the det
motors from the BB and Avenir.  Yea or nay?  I had been told
by others that there are some differences in the internals.
Block casting (amazing design) is the same?  Crank? Rods?
Etc.? If the "R" motor is the same stuff, assuming you put a
fat head gasket on to lower compression, is there any reason
not just to pump the stock motor?
Sorry, may be dumb questions - I know a bunch of you have
been here/done that; I'm trying to learn enough to decide
how to go about doing this, do it right the first time (as
much as possible) and not toss $ I don't need to.
THANKS