Back when I decided to do this, I had the same challenges as you.
All 4R's up to 98 had the split 3's (that's yours), after that, the LTD's came with a few options, the most popular being the "5 stars". As you're aware, the split 3's won't fit over the 231's but the later LTD rims all fit over the 231's.
I did some research back when I was wrestling with decision, and found a few guys that went with the 1/4" spacers. However, on the advice of others and making some measurements, I decided the 1/4" spacer was a bad idea. There are basically, 2 types of rims-- hub centric and lug centric. Most stock rims are hub centric where the rim seating and weight bearing are provided by a 1/4" lip on the hub which mate with the rim. If you put a 1/4" spacer on the hub you loose this weight and alignment point. the entire weight will now sit on the lugs--- which the stock rims weren't designed to do. It's probably fine, however I go off road and I didn't want to be the one to discover that this became a weak joint! As well as loosing this hub contact, you loose 1/4" lug thread depth. Not a great idea seeing that all weight will now be carried by the lugs! Others use 1.5 wheel spacers which is a totally different matter. These are designed with the hub lip and they utilize their own lugs. I didn't want to go with spacers, as there has been a couple of spacer failures (do a search on this site).
The other option--- grinding the inside of the rims--- really not a great idea...
I ended up replacing my rims with newer LTD's---- I paid $175 for 4 and sold my split 3's for $150. I refinished the new rims and painted them a colour I wanted.
I strongly advise you read through BFAD write up on this---- if you're interested in the 199's he mentions them in the thread. This is the gold standard on this conversion.
http://www.toyota-4runner.org/3rd-g...ements-231mm-13wl-tundra-calipers-rotors.html
Good luck!