There's a better (and less expensive) "free up a stuck engine" chemical than MMO. Pick up a quart of acetone and a quart of cheap, house brand automatic transmission fluid at WalMart. Mix them together. You'll have half a gallon of penetrating oil that rivals KROIL for rust busting ability. Put several ounces into each cylinder, let it sit a few days, drain the residue that soaks through to the oil pan and reuse it. While you're at it, pull the valve cover and soak the valve guides- - - -they can get stuck badly enough to bend pushrods if you force the engine to turn. Obviously, you'll need to drain the crankcase first. Save that "clean" drain oil to use in a pump oiler around the shop.
The 261 is basically a 235 on steroids. Chevy increased the bore size to 3 3/4" from the 235's 3 9/16" bore, added some beefed-up connecting rods, and on some engines, they used a bigger, high flow thermostat with a bypass system to avoid hot spots in the cylinder head during warmup. The crankshaft is pretty much identical to a 235- - - -same stroke and firing order. 261s ran a solid lifter camshaft, the same one used in the 53-54 Corvette six. It's slightly better than the 235 cam, torque-wise. A good substitute for the discontinued 261 cam is the Melling CCS-1- - - - -same intake timing and slightly more exhaust duration for better cylinder scavenging without causing a lumpy idle and erratic intake manifold vacuum at idle. With a few custom parts and some serious whittling on the crankshaft, it's possible to get close to 300 cubic inches and almost 10:1 compression from a "261"!
Jerry
