Keeping reliability and durability in mind, what would be the largest practical displacement LTx engine that could be built for the street using off the shelf parts, no exotic machining and retaining the LT4 heads and intake manifold? TIA...

Keep in mind my cubic inch calculations is with the common 4.030 bore but most Gen II LT1 blocks will bore 4.060 without a problem.

3.75" stroke cranks (382 inches) are a dim a dozen and require little to no grinding on the lower water jacket depending on what rods are used.

3.875" Stroke cranks (395 inches) are getting almost as common and require a little bit more grinding. 1.050 compression height piston has proven to be relibale in most street cars with good machine work but your getting tight on the ring stack.

4.0" Stroke cranks (408 inches) will fit with even more grinding on the lower water jackets of the block. Depending on the rod used you should pour the bottom of the block because at 4" you can hit water pretty easily. Some people use small journal (2.0) rods to give more clearance but with a 350 main and common imported 4340 forged cranks - this is not a good solution in a high HP car because the reduced journal overlap makes the crank to weak IMO. A 1" compression height piston can be made reliable on the street but your going to really have to have good machine work on the cyls and they will be custom pistons as I don't know of any off the shelf small bore pistons with a 1" CH.

4.125" Stroke cranks (420 inches) are availiable that can be made to fit. You will have to grind the main journals down to fit in a 350. It will be expensive. You will have to pour the block. Again custom pistons and bigger dollars.

A 3.875 stroke motor is probally the biggest you can get using all off the shelf parts. If your willing to spend $650-$700 (or more depending on the mfgr) for custom pistons and pour the block to the bottom of the freeze plugs you could easily go bigger.
Will

Now with the new Dart SHP LT1 block that Dart is working on it will make building a large cube LT1 easier. 427 cubes anyone?

When the Dart block comes out, it's on hold due to the economy , it will make it a lot easier to build a big cube LTX

The most practical is a 396, you can go to a 401, but you'll need the block sonic checked, and that's the last time you'll be able to overbore it.

A 383 is a lot cheaper to do.

I went with a 383 so the rings will seal a little better for my blower.

Explain the concept "pour the block" please. Sounds interesting.

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Bingo.

After this last build, I'm done with OEM blocks. It's Within a few hundred dollars in the end to go absolutely nuts crazy machining and modifying a stock block, when you can just buy a dart block and be miles ahead.

You can stuff a 4" stroke crank in a 4.155" bore Dart (gen 1) block for with ease for, like I said only a few hundred more than it costs me to find, check, machine, etc a 400.

Why spin a small cube block to 6800 when you can make the same power on a big cube block at 5400 without sending a valve to orbit.

-- Joe

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Street fill of hard block.

Usually fill to the bottom of the core plugs. The results are proven, but on a street car it scares me because the bottom of the cyls are not getting any cooling, so on a motor run for an hour in traffic, you can have some pretty serious issues.

Many have had luck, few have reported problems. Benefits don't outweigh risks on a street car, IMO.

-- Joe

BTW here's how thin the walls are on an LT1



block pouring is fill added to the bottom of the water jackets. When you clearance the block with for large cranks you can get really thin, or even break into the jackets

I have never been a fan of taking an OEM SBC out .060.