If space is really tight, could you use a separate microbubble resorber like the Taco or Spirovent
? Most models of the Spirovent can take an expansion tank screwed right onto the bottom -- or you can plug that port and stuff the expansion tank nipple-side up
on a distant spur, as long as the spur slopes up from tank to circuit so it doesn't become an air trap.
Pressure tanks don't much care where they are located, but you do want to keep the air out of your system. A resorber needs to be located
very close to the hot side of your system -- just after the coolant leaves the engine but before any water heater exchanger. In my solar hot water system, the resorbers are not at the highest point but do scrub the bubbles very effectively anyhow.