Nuke bag is probably the way to go.
As infres pointed out, you can probably keep the butt-pack, as technically, "by the book" it's not a permanent part of the webbing anyway, you're supposed to remove it when you're wearing the ruck, or when conducting attacks. Now of course... no one does that... and it was a silly idea, the ruck simply should have been made to accomodate the butt-pack, but still... if you ask for one, you can probably get it.
I've got an assault pack from tactical tailor myself, nice pack, bullet-proof (Figure of speech, pack provides no ballistic protection) construction, plenty of space, and MOLLE straps to attach pouches if you want. There's a pouch inside to accomdate a camel back, and a hole on either side of the top to run the camel back through, or a radio antenna. Only real major flaw is the shoulder straps are a bit thickly padded, but you get used to it. There's a company in BC selling them in CadPat occasionally on Ebay, but you can also buy directly from tactical tailor (Though not in Cadpat)