Flavio, I mistakenly thought that you wanted to use the 90/5.6 Super Topcor ER on 4x5. It should be fine, questions of practicality aside, on 6x12. It has more-or-less as much coverage as the 105 mm and shorter lenses I considered getting when I went 6x12. Since you're building your camera, you should be able to make it to accept lenses on 80x80 Horseman boards. That would solve the practicality issues I raised.
The 90/6,8 Optar and 88/6.8 B&L are very similar. They and 90/6.8 Angulons have much the same coverage. If I were looking for a 90 to use on 6x12 -- I'm not you, our needs and situations are different -- I'd avoid all three. FWIW. I have two 90 mm lenses that will do for 6x12. 90/6.8 Boyer Beryl (= Dagor) and 90/14 Perigraphe VIa. The Beryl is uncommon, the 90/14 is probably the most common Perigraphe VIb. Can't recommend either to you because of cost considerations, and didn't get either for my move to 6x12.