Rebuilding a 301 diesel ; how many hours ??

That is going to depend on how much machining of head , crank, or block is required, what parts you will have to wait on.  We (2 grown men, both mechanics) tore down and rebuilt a 190XT in a weekend, just two of us, but we had all of the parts in hand ahead of time, no grinding of the crank or head, just had lots of blow bye from a bad air filter.  If memory serves we had the engine out of the tractor in 4 hours, engine tore down and stripped in another couple of hours.  Started washing parts that night, had everything but the head clean by 10 o'clockish.  Next morning started early, ground the valves, cleaned the head, had the engine back to gether by around 1-2ish.  Tractor running by suppertime, and out of the shed with all tin back on by midnight.  Long couple of days, but we had to keep the tractor in service, it was hay season.

I,m not real sure what exactly is going on inside it yet , rattles , but when it heats up she has a knock down low , could get expensive .! ???

We figure 50 hours for in frame but we do it  all in-house including head and injector reconditioning and con rod bushing replacement/honing. Also includes a couple hours for resurfacing the o ring bore in each cylinder bore of the block. I estimate an additional 11 hours for out of frame which covers time for cam bushings,front & rear seals, injection pump idler bearings, hyd drive bearings, clutch, torque tube seal replacement and other sundries. With luck it could come in several hours less but I try to prepare the owner for something other than a best case scenario. Nobody has ever objected to the invoice being less!  Bill