May purchase 65 Barracuda

There is no way to verify "numbers matching" as was said. You can only verify that it was supposed to have what's there (data tag) and the dates on various parts that would be acceptable for a car built when that one was.
Any mods can be done. As an early A some thing s cost a bit more because it's smaller and tighter than later A bodies ('67 and later). Because the "highest power" engine was only a 275hp engine, the rest of the driveline is weaker than later As. Small bolt pattern factory stuff only - but aftermarket parts in key areas allow for larger bolt pattern wheels/tires. Wheel wells are small - so expense may not be worth it if you're going for size.