You can use a Vega piston. I am sure you could get some forged ones made. Here's what Dutra says.
"You don't think a 4.15" long-stroke engine is where it's at? You want a fast-revving, lightweight engine? How about a 210 CID low-block 170? I built one of these for my '68 Hyper-pak equipped Barracuda drag car. This was done using a 198 crankshaft in the 170 block with a 0.100" overbore (3.50" bore). I used Chevrolet Vega 144 CID pistons on 170 connecting rods, which gave 9.2:1 compression. Sounds easy? It's not, because the longer-stroke 198 crank does not quite clear the 170 block's crank pocket. To make it fit, I ground metal off the crankshaft counterweights and a small amount of metal off the bottom of the block's cylinder bore area. Most of the crank clearance grinding work was done on the first and last small crankshaft counterweights. Approximately 0.300" had to be trimmed off these counterweights so they would clear the bottoms of #1 and #6 pistons, solving the interference between the crank counterweights and the bottom of the piston's pin boss area. In order to rebalance the crankshaft after the modification, some weight-reduction holes had to be drilled into the large center weight in the middle of the crank, 180 degrees opposite the work done on the end counterweights. After this, everything cleared and was rebalanced with no added heavy (and expensive) Mallory Metal. This engine runs well and is 50 lbs lighter than a 225! It's proved a strong engine with no problems in five seasons of 7,000-RPM-redline drag racing. If I were to guess at a weak spot in this engine, it would be those Vega pistons, but so far they've held up.
Update: I did end up pulling this engine out of the Drag car after its fifth year of action. The oil pressure was starting to drop and there was a "metal-to-metal noise" coming out of it. Upon tear-down inspection, the noise ended up being the windage tray. It had developed some cracks, shifted and was hitting a rod. The oil pressure drop was due to a "trashed" oil pump, which had sucked up "rock-hard" pieces of nitride hardened roller camshaft iron. (That's another tech. article.) Bottom line is that this LG 210 engine is still "alive and well" with a new flat tappet cam, oil pump and a repaired windage tray."
Here is the entire article. You might enjoy reading it.
Stroking the Slant 6