Pre 64 Model 88's have the cut checkering which many like. They had the issues with the magazines, but worse were the triggers.
Post 64's have the pressed checkering, which many do not like. But they did have better triggers and better magazines. The ones with serial numbers higher than 189000 have the most improved triggers (and were recommended by gunsmiths in the US who were specializing in Model 88's).
I agree, your rifle as it sits should fetch $650-750 plus $100 for the magazine. Yours does have a very nice stock.
The Model 88 in 358 was only built pre-64 (1956-62) and is the rarest of the 88's. Expect to pay significantly more for these. The Winchester book (2016) I have says to add 300% to the value of a 308, for the 358. (200% more for the 284), but does not give any difference in value between pre and post 64 88's.
Reboring to the 358 will not make it worth more unless you find a buyer looking specifically for that. No different than a used car with an expensive stereo does not add to the value of the vehicle, unless the buyer is willing to pay for it.
As for reboring your barrel to 358, it isn't just that simple. There are changes not only in the magazines between the 308, 284 and 358, there are also differences in the internal action parts' design for feeding the larger caliber cartridges (358) and the fatter cases (284). If you rebore or rebarrel to 358, you will need the different magazines (expect to over $200 and probably more like $250 for these if you can find them; aftermarket are a bit less) and the action altered (more costs) in order to get the larger caliber to feed reliably. I found this out with my rebarrelled 88 to 338 Federal. It is still being worked on the alter the action for reliable feeding. Should be finished soon.