With 330mm bellows you already are close to 1:1 as 360mm would be theoreticly 1:1 with a 180mm lens.
You could try a cheap close-up lens from ebay or a pricier one with more quality, but if you will get the quality you want this way: I doubt it.
You get one that you could use for an other format and use step-up rings to fit (largest filter-size and then adapt to the other lens), in that way your money will not be entirely wasted.
If you want real close-up quality you will have to look at a 300mm or longer lens on 5x7 inch and a diferent camera like a Sinar......
An other option would be cropping your 5x7 picture down to 5x3,5 inch (or there abouts), it would be te cheapest to start out with.