Thanks for all the comments.
As to why, it is predominantly just a hobby.
-I read that the while the Pentax 6X7 lenses endure, the bodies can be troublesome, moreover the used bodies still fetch ~ $500 and they are heavier than this one.
-I wanted a real simple robust camera, the body is such the back balances the lens and forms a robust base. It can be sat on the ground or in the grass etc with the lens and back clear of the ground.The lens has a permanent metal hood with UV filter, and no lens cap. It is carried either by its handle or in an old video bag and does not have to be babied.
-This camera has no batteries (after my dslr batteries laid down in the Michigan winter) and can be used with gloves on which is not possible for example with a dslr.
-Having the spirit level in the viewfinder is good feature, more accurate than the grid in the slr and dslr. However I need to work on a better VF than the present one.
here is a photo of the Medium Format collection so far which puts the DIY body in context.
Next job is to add a body for that 55mm, the lens board is already completed as shown.
The Computar variable focal telephoto lens can only be used on the Speed Graphic because it needs the bellows. When that lens is set to 135mm, the rangefinder on the Speed is accurate, and I can use it hand held, although it is heavy.
Note that while there is a smaller Speed Graphic, these medium format lenses will not fit, they need the 4X5 size camera to get a sufficiently large lens board for the bayonet mount.
Happy MF-ing. There is some thrill in pulling the 120 film out of the tank and seeing 8 big beautiful sharp C41 frames.
( Although I haven't got there yet!)