synchro interchange/best ones to use

you could use one OD synchro not two. The reason is that one synchro has a hub/drum with a smaller inside diameter - it won't fit on a regular main shaft. I think it's the 3-4 drum that has the smaller inner diameter but I'm not 100% sure but you'll see it when comparing both drums. On non OD transmissions both drums have the same inner diameter. You could buy a new drum from Brewer's and use the slider from the OD transmission though.