The main reason a block needs to be machined is that the rings have worn the cylinder walls leaving a ridge at the top of the bore. This will be immediately obvious after you pull the head. If ridges exist, then boring of block is required and possibly change-out to oversize pistons. Often crankshaft has minimal wear and new bearing shell inserts will restore original clearances. If good oil has been used and changed regularly, even high-mileage engines can be without a ridge or crank wear, in which case, as Bram said, new rings and bearings for the crank are recommended and can be installed without machining. Honing cylinders and fitting rings and bearings is definitely doable on your own. Me, I am buds with an engine machinist, so I let him do all this kind of work. There's nothing hard about this work, but a good machinist will accomplish properly in no time what a DIYer will require a lot of diddling and doubt as to whether everything is proper.

Also agree w/ Bram about head work. Head is easily removable for DIYer, but refurbishment of valves, seats, guides, surface requires a proper machine shop. Probably cost between $100 and $200 depending on extent of work required.

You lucky dog with M41 w/ OD! I'm drooling to get one a those in my '70 145 in place of auto tranny.

Later,
Al