Baz,
Your profile would indicate that you reside in California, yet your references to part illustrations do not match any catalog I posess. Nor have I, in all my years of doing this, encountered the term "castle rail". Help me help.
I will say that in general, taking a larger approach than just where's the rusty part? Let's just piece that in pays great dividends in structural integrity and in dimensional accuracy. The number and direction of welds needs to be considered more than simply trying to reduce the total square inches of metal replaced.
Really, where are you?
Cheers, Bill
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon