YH with #16 heads could be a 375 HP 428 from '68. YH is also a '70 455, but it would have #15 heads.
Ask for the date code by the distributor and the casting number at the back of the passenger side head.
If it's got original hardware inside, it's a pretty decent motor. It will most likely have a 4-bolt block with 2-bolt caps. (All 428 blocks were drilled for 4-bolt mains, but only the 390 HP version had the 4-bolt caps.)