I recently built up a 60cm blue anodized Alan (I believe an 80s super record model) and am having a problems eliminating a creaking noise emanating from the bottom of the frame. the bike is all vintage campy record/super record except for my wheelset (1st gen campy chorus hubs/Fir rims). I thought it might be the bottom bracket so I replaced it with a newer record sealed cartridge BB. the sound became less pronounced but is still there. the bike rides and handles fantastically, so whatever it is it's not having a profound effect on performance. I am familiar with the screw 'n glue construction of these frames and I am fairly certain it is the frame that is creaking.
I am 6'3" and around 180 pounds so the frame does flex a bit when I push the bike hard, but it doesn't seem to be any amount worth worrying about...

any suggestions as to what might be causing it and how I can fix it?

Replacing the bb would also have taken into account the crank bolts. Did you grease the threads on re-assembly? Did you check the cranks for cracks propogating for the spo\indle hole and the crankarm/spider junction? Also, remove, grease and retighten the chairing bolts and pedals. Coud also be within the pedals.