Try this for an AST. Greater capacity and uses a standard cap. COST IS $62.00 AT

http://www.pitstopusa.com/ALUMINUMEXPANSIONTANKS.asp

CAVEAT: YOU MUST PUT A 0.1 INCH DIA. RESTRICTOR AT THE BOTTOM OF THE TANK OUTLET (LOOK AT THE STOCK AST BOTTOM PORT) FOR IT TO WORK PROPERLY, OTHERWISE COOLANT WILL GO STRAIGHT FROM THE FILLER NECK TO THE LOWER RADIATOR HOSE WITHOUT GOING THROUGH THE RADIATOR.

 

where did it say what you said (about the caveat)

 

The site doesn't say that, it's what I found running my car using this AST without a restrictor. Radiator end tanks were cool but the AST tank was hot as hell. I looked at the stock AST and saw a restrictor there and figured out that out.

 

There seems to still be an element of the equation that can potentially go wrong, like a stant cap and it's stupid vent lever.