I would strongly advise you go with the Hornady Gas checks. As you probably know these are actually crimped on when the cast bullet is run into the sizer die and lubed.
The Lyman gas checks do not have a postive "lock" like the Hornady's. They are just a "slip fit."
If the base of the bullet were to be seated below the neck-shoulder junction, this could be a big problem.
I did some searching and was surprised to see all my old suppliers no longer carry the Hornady's in .270 caliber. However, Hornady themselves have them for sale on their site.
http://www.hornady.com/shop/?page=shop/browse&category_id=85a2811523312a2b50048cd45ecc5661Two more things:
1) If you plan on casting a lot, go scrounge wheelweights now, before all the Mom and Pop tire shops find out how much lead is worth these days.
2) A great source of information for cast bullet shooting is the CBA.
http://www.castbulletassoc.org/The "Fouling Shot" is well worth the price of membership.
Good luck to you getting cast to shoot well in that rifle.
It can be a lot of fun,, though at times (with my rifles anyways) it seems akin to searching for the Holy Grail.
Matt