Can a 318 crank and rods be used in a 340?

Wouldn't aftermarket rods be better stronger lighter and under $300

Sure, but the rods we're talkin about were produced LONG before you could buy something nice around every damned corner.

But to answer your question, in fact, no, there are scarce few rods that are as light as the 273 rods. In fact, there may not be any except for aluminum. That's why they were so popular. Prepped correctly and with good bolts installed, they are very strong.