Inspection 

This particular AL-1200 is one of the earlier ones manufactured-- # 23. The owner from whom I purchased it (Jan KX2B) was the original; the tube suffered some catastrophic failure many years ago, and the amplifier had been sitting around since.

On inspection (always a good thing to do to understand what you have), I found that  the 'slow-start' or reduced voltage turn on resistor was open and charred (perhaps a short on startup sometime in it's past). 

Mechanical and cooling 

After removing all of the 3cx1200 related items, the challenge became figuring out how to 'fit the 6lb gs35b into the 4lb box'. The original AL-1200 fan is good for 55cfm (according to Ameritron ), so that had to go, but that left even less room. Looking in the Grainger catalog, the Dayton 2C067 Blower has the motor partially set into the squirrel cage area; the CFM seems about right with about 80cfm at 0.2" of static pressure.  This seemed like the right blower, but it was too big to pressurize any kind of sub-chassis.  The only orientation that worked was blowing directly towards the front panel. The challenge then became to get the air to actually blow through the tube cooler. Dividing the plate circuilt area from the tube compartment was considered but discarded because of difficulty in fabrication (I don't have a metal brake). Then an inspiration -- if the existing cover's perforations could be covered, the entire RF-side of the chassis could be pressurized! Everything then fell into place.