Bell housing bolts are 3/8ths NC x 2" ? long. ARP makes a kit for chasing Coarse threads that only cleans out the crud. A tap is designed and meant to cut new thread in a bore or hole. Used as a thread chaser it will shave the peaks off the existing threads and make them loose. Makes it easier to strip 'em out.
Before you go trying to run a bolt into any hole, be sure to clean it out. Deep Creep works, CFC Brake Cleaner does too. For a brush, try a gun bore brush from an Outdoors Shop. They come in various diameters.
The value of the 400 depends on what use you plan to make of it.
For an out and out g-machine, don't even bother with a 350, go straight to stroker. The 383 has the torque; the 377 the top end.
The 400 was designed as a heavy vehicle motor; trucks, R-V's Chevrolet Impalas, 70-72 Monte Carlos. The proverbial two tons of fun.
As built, their components were selected for the purpose of making torque, not horsepower.
Contrary to some schools of thought, they can be built. A lot of them got raced to death on the dirt short tracks way back when.
The major considerations here are, having the block cleaned and magnafluxed. That will expose any external damage and internal issues. For a decent street motor forget the stock components They will give you the factory values, no more.
Once it is clean and checked for cracks, have a bore gauge taken to it to check the size and shape of the cylinders. That will disclose any previous overbores and tell you if there is room to grow left.
For the reciprocating assembly, Scat, Eagle, ?? This is a homework thing. Their catalogues offer all the info you'd need to pick a combination that will do what you want. They also have 800 numbers for questions.
If you elect to convert the heads into paperweights, then be sure that whatever replacement you acquire is compatible with the steamhole circulation design of the block.
All the rest, cam, valve train, intake, etc, are whatever will work best for what you want.
As anyone here can testify, if you want a good motor, you pay good money and buy good parts and use a good machine shop or builder to assemble it. This is one case where Cheap in equals Cheap Out.