It should be possible to remove the timing cover without removing the heads. The head gasket does go over the lower timing cover BUT it is just where it seals to the head and if careful it shouldn't cause a leak on re-install. A small amount of RTV or anerobic sealer might be a good idea on the re-install though.

Note that the main crank bolt has to be removed and it is VERY TIGHT. I couldn't budge mine with a breaker bar and 5ft jack handle. The breaker bar would just flex and not move it. I am sure if I went and got an even stronger breaker bar I could have gotten it. It just wasn't going to happen without tipping the engine over on the stand...

The reason to remove it is more to do the transfer pipe seals than the lower timing cover gasket. From what I understand the lower gasket is a different material than the upper ones and is less likely to leak.

On my engine change the lower timing cover gasket and front crank seal ended up being the only ones I didn't replace....

I would also do the valve cover gaskets and spark plug tubes with the upper timing cover gaskets. It is probably possible to do the upper timing covers without the valve cover gaskets but it makes the valve cover gaskets much more likely to leak in the future (they overlap in a similar way to how the lower timing cover overlaps with the head gasket).

If you have the valve cover gaskets off and you are up for a big job then the valve seals should be replaced if they are original...

-Rich