Have you talked to your machinist to confirm the price would be the same? Usually they charge by how much they cut, I'm not sure I've seen one yet that didn't (keep in mind, there aren't many machine shops round here, so my experience is limited). Another thing: Your dad had a decent point, but you also said the forged motor would be built even higher, so that nixes that. However, the larger the bore, the weaker you will make the cylinders, and in turn the block. I wouldn't set a specific bore size, just whatever is needed. What power a bigger-than-needed bore might make would be counteracted when the bore distorts and is out of round (round bores make power)
I'm not trying to shoot holes in all your plans, I'm trying to help you make them bulletproof! :D
Oh, and your swapping plan is pretty similar to mine. I swapped out my original motor (which I believe is forged, haven't been able to confirm. Engine stand is in use!) for a built cast motor. This leaves me with a forged(?) motor to build for... whatever... if I so choose. Or I may jump the gun and drop in a roller 360. Who knows.