Mid-1950's through early 60's school buses are a good source for 261's, if you can find an old-time salvage yard that would buy them up for scrap value. You're also fairly close to Canada, and some of our stovebolter buddies up there say Pontiac car engines are still available at times. Those engines also have the option of running hydraulic valve lifters, since the passenger car 261 blocks were bored with a front to rear big oil passage through all the lifter bores.
Jerry
