Before you get hung up on the compression ratio, you mentioned that you wanted a long block and then do a cam swap.
You should determine your compression ratio after you select a cam. A mild or factory cam is going to require a lower compression ratio. This is because the intake valve closes sooner, and thus raises the dynamic compression ratio. This is why the L31 had a CR in the high 8 and low 9 range. For comparison larger/performance/radical cams have an intake valve that closes much later after BDC. At low speeds this allows air pressure to bleed off, but at higher RPM's there is a ram effect of cramming more air into the cylinder.
Also you mentioned keeping the factory computer with the L31 long block. I think there is some leaway in using a slightly better performing cam without having to modify the computer. I am not positive, but I think the cam's LSA needs to be pretty close to the original. So that all goes back to how much performance do you want/expect, and if you want a lot, you better factor in a computer retune to work with all the performance goodies.