This after market screw on bolt modifications are not very secure and IMO are a waste of effort. Bolts need to be TIG welded.
http://maxshepherdboltproducts.com/
To reborn and chamber, I recommend Jesse at www.35caliber.com . I’ve had three done by him and couldn’t be more please. Reboring allows you to use the original barrel, therefor no stock work is required.
Typically not. The 243 is a short cartridge based on the 308 and the 280 is closer to a 30-06. Does your magazine box have a spacer in it? If so, it will have to be removed. Some are pinned in place, others maybe screwed to the magazine box.
You can see the difference in overall cartridge...
You need to consider feeding issues with any cartridge outside of the original case. Most milspec M98’s were designed to feed the 7x57 or 8x57 case. The follower can be changed to facilitate some case size changes along with magazine block spacers for shorter cases like the 243, 308, etc...
You have a decent start on a sporter and have made some good choices. I would do one thing different and that is drill and tap for a receiver peep sight and you won't have to alter the safety or bolt. Peeps are very handy and accurate, especially if you are doing a short barrel 75 yard brush...