Leave it as 281 or whatever .010 or .020 over winds up being if you even need to bore it. If your bores check good, get it honed and go. With the extra cubic inches you would see a little more power... a little.
Other advice, do the whole thing at once. If your current motor is running great now, leave it alone, go find a second modular block from wherever and start building that on the side. Get the whole thing built and drop it all in at once, ported heads, new cams and all.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.