"To be road legal in the US the engine must be the same year or newer than the chassis."
You sure about that? Is that nationwide?
<looks online>
In fact it is:
https://www.epa.gov/sites/production...ngswitch_0.pdf
Second paragraph, second page. Spells it out in such a way that includes engine swaps. However an exemption is offered, in that a swap is permissable if it can be demonstrated (or reasonably assumed, according to the text) that a swap will not make emissions worse as "described in Memo 1A," whatever Memo 1A is. No link. But if you do a swap and run it through someone's exhaust analyzer and whistle up better numbers than stock, that might suffice to win you a pass on an older-into-newer swap.