Gold Founding Member
Join Date: Aug 1998
Location: the Seasonally Frozen Wastelands
I've got six GM tall deck Mk IV 427 truck blocks, with two or three casting numbers. The one complete engine has two spun rod bearings, and a mallory unilite conversion in the distributor. I paid $250 by the time I picked it up and drove it home. (300 miles) I consider that a steal of a deal. Most are not that cheap. I've had an offer of a free 366, though.
There are MAJOR differences among these TD 427's in terms of crankshaft clearance "cast in" to the bottom of the cylinders. Three of my blocks have SIGNIFICANTLY more clearance than a 454 passenger car block I compared them to. Others have the same or less clearance.
Crank-to-cam clearance isn't an issue with big blocks. Some of these blocks will swallow a 4 1/4 stroke crank without grinding. (That is according to others, I have not tried this. Looking at the clearance cast into some of my blocks, though, I have NO trouble beliving this.)
The Tall Deck 427 was built with 4-bolt mains, forged crank, and the heavy-duty 3/8 rods. If the price is right, that alone could make it worth buying the TD, if only for parts. The crank will be balanced for HEAVY pistons, though.
I've seen a wide range of heads used on tall-deck blocks. Large oval ports (2.07 valves), round ports (2.07 valves), and round ports with TINY valves, perhaps 1.94 or thereabouts.
In short, inspect the block casting for crank clearance, look at the heads, and see what it's worth to you. The taller and wider block can give you problems with A/C box clearance, brake booster clearance, header fitment, and hood clearance issues. All of these are fixable, depending on what you're willing to do.