I have an 87 d150 with a slant six. the truck I believe has 40,000 original miles and has an intermittent lifter tick. I have tried combinations of sea foam, marvel mystery oil and atf in the oil which seem to help but won't cure the problem. the lifter seems to pump up best when the engine is HOT but i still think there is some excess valve lash. can a single lifter be replaced without removing the head? how about a slightly longer pushrod or is there a way to tighten up a single rocker?

Yes. If the engine has the correct hydraulic (wide valve cover) head, you can "fish" down and pull the lifter(s) out thru the push rod hole. There is a special tool that makes this job easier.

There is not much you can do to adjust lifter preload when you have non-adjustable rocker arms.
Replacing the lifter(s) re-torquing the rocker shaft and head bolts and using adjustable pushrods or lash caps are your only "don't pull the head" adjustment options.
DD