We are kinda getting away from bargains here but your questions and their answers may be helpful to buyers looking to buy Shopsmith tools.
Dusty, you asked,
Is there any reason why I could not build up a <ark 5 casting with current version parts? Is everything compatible?
Over the years changes for the Mark 5/V have been made backwards compatible so you can have a Mark 5 "A" Headstock from 1954 or 55 with current parts in it. Some can be changed individually while others as assemblies. You can replace the quill return spring with the current one going back to even the first Model 10E's. One change may necessitate another such as changing from a Gilmer Drive to the Poly V Drive also requires changing the Quill Assembly. Upgrades can be made to the Mark 5/V such as the Table System or Power Pro DVR Motor.
Dusty you also asked,
If I had done that, what would I have? A Mark xxx ??
If you only make changes to the headstock it would still be the same Mark 5/V model as all Mark 5/V Model's use the same headstock. If you change a Mark 5 or V with the original table system to one of the larger Table Systems of the 505, 510, 520 or Mark 7 what would it be called? I have a Mark V 520 that has been completely upgraded to a Mark 7 which is what I commonly call it even though the serial number (date code) on the headstock is 09-11-01 and the Mark 7 came out in 2010. You can see that in the list under my name below. That headstock is one I put together with the parts that came with the Power Pro motor upgrade and purchased parts including a "C" headstock shell. I kept the original headstock as a spare/backup. The Mark V 520 listed below was originally a 2006 Mark V 505 in which tables have been upgraded. For the Mark V it is the difference in the Table System that makes it what model it is but it is helpful to know it's history. A lot of information about that can be found at the link below.
https://forum.shopsmith.com/viewtopic.php?f=7&t=24963
