How do I know it's a 383

[79 USA 1]

If it is an older 2-piece rear main block, the balancer will have a scallop in it if you can get under there to see.
https://www.summitracing.com/parts/flu-720111/make/gmc
I know this is not a stock balancer but it's the first pic I found.

Found these pictures on the interwebs.

Harmonic balancer has scallop like this


Flex plate will have a balance weight similar to this.

 

[565bbchevy]

These days no one really uses 400 SBC cranks for 383 strokers when you can get an Eagle or Scat crank that is cast or forged for a reasonable price plus they can be external as well as internally balanced which would eliminate the telltale weighted flexplate and balancer.
Probably the best bet would be pull the pan it might have stampings on the crankshaft and would probably have 6.0 aftermarket rods.

 

[Dayzedandkonfuzed]

Unless the seller can prove it, buy it as if it's a 350.

X2

Everybody around here says they have a stroker, or a 327. And I can't count how many 350's I've seen with a timing tab at 12 o'clock

 

[JAMES L EUBANK]

Thanks for the tips. I'll look for all of those items. If possible, I'll get it on the road and see how she performs.

 

[fleming442]

It will have 5 or 10 valve cover bolts, an oil fill tube behind the water pump, and the distributor turns counterclockwise

 

[CaliWagon83]

I’ll echo what others have said here. If there’s no supporting paperwork, assume it’s a 350.

Honestly, it’s all in the tuning and setup. Yes, generally, 383s are torquier than 350s, but like any other kind of engine, there are 350s that run like raped apes, and 383s that run like dogs. If you drive it, and it meets your “butt dyno” expectations, I’d make him a reasonable offer on it.

If he keeps insisting “But it’s a 383!” I’d tell him, “Look buddy, I’m not saying you’re a liar, but with no paperwork or evidence, I’ve got to assume it’s a 350, so that’s what I’m going to pay you for.”

 

[Ribbedroof]

Unless the seller can prove it, buy it as if it's a 305.

Fixed it for you!

 

[Ribbedroof]

Seriously, without receipts and a way to reconcile them to the unit in question, it's a core. Who knows what's really in it...might be a krylon rebuild, or might be the real deal. How lucky do you feel?

Remember, every SBC came out of a Corvette, and is a 350...even when the deck stamp codes to a 267.

 

[Doug Chahoy]

When I got my 383 I had to get a stroker oilpan and said balancer

 

[olds307 and 403]

It will have 5 or 10 valve cover bolts, an oil fill tube behind the water pump, and the distributor turns counterclockwise

That is my favorite 350 to stroke as well.