6lb ! My shoulder hurts just thinking about it
. You need to check the design weight capacity of the heads. You need a heavyweight "video/fluid" head. I would also recommend you get one with a pan bar that's at least as long as the rear overhang of the scope. I find it hard to smoothly pan/tilt a big scope with a short pan bar. The Ulanzi one is too short and the Small rig one looks too short, but may be extendible. The Manfrotto ones are ideal.
With such a heavy scope you might need a head with something like an Arca type sliding plate so you can balance the scope so it doesn't tip backwards or forwards when you let go of the pan bar and forget to tighten the tilt lock.
I also prefer an angled pan bar and separate pan and tilt locks, but you need to find out what your own preferences are, preferably at a shop/field day that stocks Manfrotto heads. Take your tripod and scope and try them out. You could then check if your scope is noticeably nose or tail heavy. I don't know where you are in the "North East", or if that's UK, US or someplace else ;-), but if it's UK then In Focus has field days at Hauxley Nature reserve in Northumberland. Others may chip in with "local" dealers.
