No problem putting them in at all, drilled and tapped (threaded) the holes and they screw right in and seal up with no problem... The side walls on a 55 gallon drum depending on where you drill can be 1/4" thick or more so IMO it's best to tap them vs forcing them as they will shear off if you force them...
For that barrel I used a 1/8 (NPT) tapered plumbing pipe tap as horizontal nipples have a unique proprietary thread, ideally a metric 10mm x 1.0 (tapered) would be ideal as that is the thread used on vertical nipples... But at the end of the day the difference between a 1/8" NPT and M10-1.0 (tapered) is insignificant in this application as they are nearly identical, and even in metal you can force fit between the two if push comes to shove...
You can pick up the taps for a few bucks on Ebay, you really don't need a tap handle if going into plastic, you can just use an adjustable wrench or locking pliers...
Or you can do what I did and just pick up a cheap tap/die set as it's one of those tools that if you have it you will likely eventually find uses for it...
This is the cheap set I have, it's all imperial screw size except for the single pipe tape, kind of idea in this application...
http://www.harborfreight.com/hand-tools/tap-die/40-pc-carbon-steel-sae-tap-and-die-set-63016.html
Only $15.99 or $12.79 with a 20% off coupon printed (or just download it to your phone and show the cashier) from this site, you can even plug the coupon numbers in for online purchases...
http://www.hfqpdb.com/ and while you are there at the site print (save) one of the free item coupons as well and get even more value with a free item...
As for the hole size to drill 11/32" is the proper size for a 1/8" NPT but you can get by with a clean 3/8" hole or a slightly enlarged 5/16" hole, aka run the 5/16" drill in and out a few times...