Not quite so easy to get it right. Could just add it on, drilling two holes in the gas tube in just the right spot, and let the gas block hang in the air, but it really needs the support of the stock (like it has on a QB79). So it's better to shorten the gas tube of the 78, add the gas block, inlet the 78's stock, glass bed the bottom of the block to the inletted stock, then add the two supporting stock screws into the block. It's not hard work, but it's detail work.
I like it...gives a full length fore arm. Does add weight out front, but it hasnt been a problem for me (here they are running on HPA, but small sized co2 tanks will fit just as well): Click on picture to enlarge

Archer is out of stock on the gas blocks...might see some coming in during the summer.