First: I assume you have a loosefooted main? I assume the the blocks you describe are sitting forward of the outhaul connection or strap? If so...

o Run the reefing line from the end of the boom
o Through the block
o Up to the reefing cringle
o Down around the boom; and
o Use a bowline "slip knot" so you cinch the thing down on the boom

http://www.sailmagazine.com/boatworks/update-your-reefing

We use this method without a reefing car. I don't think the reefing car will really be an issue UNLESS you bind it up with the adjusted reefing points "clew".

The idea is to pull the reefed clue back and down.

As for the diameter of the line: I think the issue is threefold: tensile strength and stretch of the line; ease of handling (hands and line stoppers rangch); and, chafe.

A 3/8" line on a 27' boat shouldn't be an issue (I would guess) from a tensile standpoint. The stretch is relative -- you probably won't be setting that reef that agressively. If you really are concerned with sail shape and trim, then you could use a lower stretch line -- or up the diameter.

Finally, the size of the line depends on how you set and hold the reef in position. If you're unloading the sail under way, setting the reef before you go sailing (not in the thick of it), etc. -- then let your hands be the guide. Then there is the issue of the stoppers -- line stoppers have a range. If you have a line stopper that is set for 7/16 - 1/2", then you might not be happy with the 3/8" line. You should check that.

The last issue is chafe. If you set a reef offshore and are, let's say beating to windward for 24 hours or longer, you should be concerned with chafe on the line as it goes through the cringle. Bigger is better with regard to chafe (in general). In your case, I doubt you'll be doing that, but again, that's for you to answer. From your general description of your sailing, I'd just be aware of the issues.