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As you have already ascertained, the block casting (with a date code of September 1969) is a 402. The reason for your “outrageous” compression ratio is that for the 1970 production year, the lowest compression ratio for the high performance 402 was 10.25:1. Most of these ended up in SS Chevelles and SS Novas.

There were three versions, specifically;
330 HP @ 4800 RPM, 410 torque @ 3200 with 10.25:1 compression ratio
350 HP @ 5200 RPM, 415 torque @ 3800 with 10.25:1 compression ratio
375 HP @ 5600 RPM, 415 torque @ 3600 with 11:1 compression ratio
The 375 HP was solid lifter only available with a 4 speed and no A/C

Your heads are not from this engine. 3872702 heads were only produced in 1966 and was used on 325 and 360 horsepower 396 engines. These are oval port 253cc port volume, 2.060 intake / 1.72 exhaust. Probably one of the best big block heads to come out of GM during the “muscle car” era. In stock form (per Chevy High Performance), they flow @ 20" at .400 lift 214 intake/135 exhaust – not too shabby.

Your pistons are OEM – GM Part Number 3969974 is for factory forged 4.125" pistons
These are a standard bore 402. They could well have been made by TRW under an OEM contract.

I’d look for another cam and intake manifold as well – roller on the cam and Performer RPM or similar on the intake.

You did not say what shape the engine is in, i.e., miles on it or if there’s lots of obvious wear. If things are pretty good looking you could go .030 over with a tamer piston, have the heads done and perhaps install a more contemporary roller cam and have a great engine.

A suggestion for you – if you ask nice, Pat McCarthy will give you more advice than any ten local “experts”. Pat is one of the premier Chevy big block builders in the world – trust me. The fastest way to get ignored on this site is to get snippy or cop an attitude. Most of us here have other jobs and it sometimes takes several days between posts. Try and engage Pat, Jerry, Techinspector and others and CHR can be a great resource.

Regards,
Glenn

What Glen says.