how much does 1971 340 stock intake weight

The Performers are cast with large runners & small ports.
They are easy to open the port to a 340-360 port size, you only have to in about 3/4 of an inch.
I always port match a Performer when it's going on stock 340-360 intake ports.

A Performer would be a power loss compared to a stock 340 intake. Not worth the time or money to switch.