Pontiac of Canada used a version of the 261 as its base 6 cylinder. I have two of those blocks, one of which is in my 57. They are a little bit different. For example, a Chevy 59 261 generally is found with full flow oil filter and the larger oil filter lines, the Canadian Pontiac version still used a bypass filter and the small filter lines, even though the blocks were cast in 59 for use in a 59 car. So it is a little bit of a mash up. The Canadian Pontiac blocks also generally had the 55-57 front side motor mount arrangement cast in place, and some had both the front and side mount arrangement. I doo not have any later Canadian blocks but I have heard that those engines did not get the full flow treatment during the time they were used in the Pontiacs. Chevy truck 261s are somewhat all over the place. I just looked at a 60 block, no 55-57 mounts, and a couple of years ago I saw a 59 chevy with both. I have a chevy 261 block that I pulled from a oil truck 30 years ago buried in the back of the garage. I had a main cap split on me 25 years ago when I was putting it back together. Full flow lines and both sets of motor mounts so I think its hit or miss. I will look at that block because I am not sure whether that block is drilled for hydraulic lifters or not. My gut tells me yes only because the 55-58 steel line is not present and so there must be some provision made to feed the rockers. It would make sense to drill the galley, even though the motor originally came with solid lifters (and had the st874 style lifter in the motor when I originally took it apart in the 90s).
I used mechanical lifters on the new motor. Unless you have some real GM nos hydraulic lifters, or some hydraulic lifters that are like 20-25 years old or older, you may run into the lifter issue thar has been somewhat troublesome for a couple of years now, namely, brand new hydraulic lifters collapsing at break in or shortly thereafter. That and I was building a very modified motor with a fairly aggressive Schneider cam so mechanical was it. Also, while some guys like to garden, cut grass, put little white balls into a hole, as a form of relaxation, I myself find adjusting valves, performing a tune up, oil change, or whatever on my old stovebolt motors the ultimate in relaxation, lol.
A friend of mine owns a machine shop. He had rebuilt a 51 or 52 powerglide 235 for one of his customers. Used new lifters. Had issues with two different sets. I gave him a set of nos gm and what do you know, not a single issue.
