To keep with the Quad 34/306 vibe and be period correct the Spendor-made Quad 10L would be perfect as they were designed to be part of the system, but they are fairly scarce. The closest modern equivalent would be the Spendor 3/5R. I’d be more inclined to hunt down slightly larger Spendors of the period, say Preludes, or Rogers LS7s, something like that. Obviously earlier ones would be fine too.
The Chinese made Quad 11L seem to divide opinion, I rather like them and they can be had very cheap. They look very similar to 10Ls, but are ported and have different drivers.
PS The 306 will drive ESL63s to reasonable levels too, they are the speakers pictured in the manual! To be honest the 306 will drive most things, it is a good punchy 50 watt amp, so pretty much anything is on the map unless it is crazy inefficient, reactive, or real heavy rock levels are required!