Oilfield Trash:
Welcome to the Stovebolt. You will find everything you wanted and more on this site. There are many very knowledgable Stovebolters on this site.
With regards to disk to drum conversion, can't help, since I haven't done one. However, I have done the power steering conversion. Both were on factory V-8's, so the power steering pump brackets were readily available. They are different on the straight sixes. There are many power steering adaptor kits available. Start with Classic Performance Products, Classic Industries, Chevy Duty or REZ Engineering. They are virtually the same components and price range. I have also seen them on eBay from time to time.
With regards to a seat conversion. There are no hard, fast rules with replacing the seat. They key is overall length, overall depth, overall height. Finding a seat that is close in dimensions to your factory bench is a matter of finding something you like that will fit. I put a set of black leather electric seats from a late 80's Cadillac in a '65, C-10. There are many options available. It really depends on your taste.
In any event, good luck with your '61 Apache and welcome to the Stovebolt.
