Lionel are by far the easiest to convert to two rail.
Railthunder
Be prepaired for something easier yet. The more recent versions of the MTH 52' 6" gondola have underframes designed for Kadee mounting. No shims are required when using MTH 2 rail trucks. The MTH gon is a pretty good representation of the PRR G-28 and the many similar AAR gondolas of the late 1930s and 1940s.
All of the MTH GS gondolas are intended for easy Kadee mounting. Petersen has also had MTH make GS gons that came factory equipped with 2 rail trucks.
For more information on MTH and other O scale gondolas see:
https://ogrforum.ogaugerr.com/t...t-car-guide-gondolas