351 you just need the swap headers, 351 accessory brackets and then either a cowl hood or drop mounts for underhood clearance (get a cowl hood). Can't be a simpler swap in a pushrod car and anyone who does a rebuild and does NOT go 351 is not doing it right IMO.
And yeah ANY 351 block will be good for north of 700whp so just get a F4-up roller block, check under the cam bearings for cracks (or just buy a Fordstrokers block/shortblock since Woody does it all for you) and go from there. 351 intakes and heads are not really much more money than the 302 based stuff and you make a metric assload more power with the 351 based motor. A mild 408 stroker will be making 500+ to the wheels and you'll come into a FAT power curve with sphincter clenching torque at a really low RPM level too.
edit: To answer your original question if you just want a quickie replacement to get you back up and running for the time being then any 302 will be good really. Best bet would be one out of an Explorer since those come with the GT40 stuff on them already. You'll need to upgrade the valve springs and swap in your stock cam (or an aftermarket one) but that's no biggie.
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 ****.