6 wheel trucks: 2 rail to 3 rail wheel sets?

What you are doing isn't very common. It will be hit or miss as 2r has thinner wheels and much smaller flanges. I'd think swapping in whole 6 wheel truck sets would be easier. Id also expect the 2r crowd to have a good guess if you could at all before the average 3r fan.

I don't know how "average" I am in the 3RO world, but I have changed several sets of 2RO wheel sets for 3RO wheel sets. I have made Walthers - and others' - trucks into 3RO units. I have done 6-wheel and 4-wheel. The wheel sets (Lionel, MTH, Weaver, etc) I find on eBay, typically. I have been pleased with the results in almost every case. 

Now, bear in mind, speaking of cases, that this is a "case by case" bit of business. Pointed axles have been no issue - in fact, more a good thing than a bad. The width of hi-rail wheels is greater than the typical 2RO wheel, and varies with brand, so some truck frame rubbing can happen. If it is slight, I ignore it - it will also wear in, with time. It does not always happen. 

Try a set; get some wheels. Get a feel for it. Lots of variations and variables, but the beautiful (and otherwise) old 2RO stuff can be a gold mine for a 3RO Hi-railer who likes to roll his own, sometimes. Often cost-effective, too.

You will seldom get modern. ultra free-rolling Delrin-bearing performance with these upgrades, but most are not bad, and none have been worse than good Lionel Post War trucks. 

@glacierrunner posted:

Looking  at buying some Walthers standard pullman 6 wheel truck kits with 2 rail wheel sets. Will there be any issues using 3 rail wheel sets in the side frames?  I've been reading that I should be using brass bushings and blunt axles. Any one have some 3 rail wheel sets that would work forsale? Thanks, Don

 

Your other issue in using 3 rail wheelsets with two rail trucks is you will have to bypass the fast angled wheels for their wheels in many cases are too wide for the two rail truck bolster. You will have to look for the postwar wheels with blunt axles at best.

Walthers used to offer their trucks with three-rail wheelsets, the castings probably have not been changed. Some years ago, I was at a small train show and a seller had several Walthers cars, one had two-rail wheels. The car that I wanted to buy was a three-rail one. He let me change one truck and swap the axles in the other. The two-rail car had two different style trucks and I wanted to retain the style on the observation car that I bought.