anything can be made to fit...how talented are you.
Seriously, measure the tunnel, as long as the tunnel will fit over the backbone you can always add mounts to the frame to make it fit (a good welder should do the job for about 20-50 bucks). IIRC the allo tank is beween 550 and 1200 dollars depending on where you shop, at the cheapest it is a bargain but at the top price you might as well have a real tank builder (like evan wilcox) build you the proper tank specifically for your bike. As far as the seat is concerned the one I see hitchcock's selling looks like a dunstall style which I know you can get for a cb750 and 550 in fiberglass from places like airtech. Alos check on ebay as there is a guy that sells a thruxton style seat (like the one in the pic) for the cb350 that would also probably fit the 450. There are several places out there that I am sure guys will chime in with just look around. Unless you already have the enfield pieces it is alwyas better to start with stuff actually made for your bike.