FWIW - I have that very same XS750 tank for my '81 Special-to-Cafe build and the MikesXS NON-vacuum petcocks fit perfectly (NOTE: there are several models listed - check the spacing of the holes in the tank BEFORE ordering) and the MikesXS gas lid for a 1976 XS650 Standard also fits perfectly.
As for the inside - I have cleaned two tanks as of now and I did one with POR15 (worked great but it is laborious) and had one "tumbled" with some aquarium gravel and a litre or two of water (also worked great and it was little or no effort). One of the guys in my classic bike club simply uses about 25 roofing nails and then he wraps the tank in an old sleeping bag and puts in the dryer overnight and the next day. He claims that works great too. He says to be sure to count the nails going in - and coming out to make sure you get them all.
BTW - POR15 is the stuff that Jay Leno uses in all of his classic cars - and if its good enough for him, its good enough for me.
Unless the tank has an actual hole in it - I wouldn't be put off by some rust or other gunk inside.
DO NOT use a product called Kreme to clean or seal a tank. Apparently, this is an obsolete product which can be dissolved by modern gasoline which often contain ethanol - and the mess it makes is pretty bad. When I started working on my 1976 XS650 all the local shops wanted to sell me a Kreme treatment - but my research confirmed that it is not good stuff.
Cheers,
Pete