First of all remove the striker band and rod and check for corrosion. Images of the rotten ones I got below.

The 18SQ beams I bought had no inserts what so ever. If so, try to find a pair of Australian 5.8 beam inserts. Think JeffS knows a guy who has a pair. Also add a 4" long 0.120" tube in the center.

My 5.5 front beam had tube inserts of something like 0.08". If you have them, change them for 16" inch long 0.120" and also add a 4" long center insert.