Some thoughts:
If I understand you correctly, you think you may use your 4x5" bellows with a 5x7" (or even a 8x10") lens. Usually, any 4x5" bellows/cone is simply too short.
Think that theoretically, at 1:1 you need to double the focal lenght to find the bellows draw/base to lens distance. That is, with a 150mm lens, the bellows draw should be near 300mm, and the base to lens distance about another 300mm. At 2:1, just add another focal lenght to that base to lens distance (that is, 300+150=450mm), for 3:1 another focal lenght (300+150+150=600mm), and so on. To calculate the bellows draw, for 2:1 add half the focal lenght to the focal lenght (150+75=225mm), for 3:1 add one third the focal lenght to the focal lenght (150+50=200mm), and so on.
Just apply this calculations for any enlarging lens. 5x7" lenses usually are in the 180-210mm range, 8x10" lenses are in the 240-300 range (WA versions aside). A 240mm lens will need 360mm of bellows to focus a 2:1 print.
Don`t know about your enlarger, but you can calculate&DIY a wide enough lens cone (if possible) to be used at a given magnification. And you can made a "box" to be used on top of your film stage... if possible. And there is the colum height. And you`ll need a good film carrier (big&heavy). A new light head. Etc. etc. etc. (BTW, the light head will take you a couple inches at much).
Let`s be realistic. Do you really need to do this? Isn`t 4x5" big enough for you? What about contact printing 8x10", or even to work on alternative processes? At the end, 2:1 or smaller from 8x10" seem too much pain, too much homemade, too much flimsy approaches to make a not so big print at all. And don`t miss that shooting 8x10" is not cheap.
Well, if you want it, that`s right. But honestly, I`d look for a "true" 5x7" enlarger instead. 8x10"s are not so common anywhere (nor cheap!).
I`d find even easier to build a 16x20" camera in order to make contact prints...
Good luck.
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