Criticize? come come now,.. surely you jest.
Rebuild is one of those words that doesn't seem to have a formal meaning & it can mean diferent things to diferent people, which is why i am always suspicious when it is used. At trade pricing, the parts involved in a minor overhaul, when done properly, will cost somewhere near that figure, while a re-"build" is normally used to describe either a major overhaul, or a recondition job.
Your ready access to skilled [& free] labour, along with the services that these people can provide [like major head work] was unknown to me [us]
To rebuild your engine as you describe for under $450 has to be a real bargain.

The costs involved in my engine "rebuild" have run to somewhere near $3000 so far [pistons, rings, head job, rods, crank, distributor, plugs, adj. cam sprocket] & i have not dismantled the short block yet. By the time it is running, it may end up costing as much as ten times your figure, hence the curiosity about the definition of "rebuild"