[Hank70SS]

Could that be 353049? That would be the original heads for that engine. If there are machine shops around you that work on those engines you could have larger valves installed, 2.19" intake, 1.88" exhaust versus the original 2.06" intake and 1.72" exhaust and have the combustion champers cleaned up. Those oval port heads will work well if the rest of the combination matches. You don't need rectangular port heads. Install a performance cam, moderate, nothing to radical. You can have the heads milled down or the block decked to increase compression but that isn't necessarily required. You can get over 400hp out of 8.5 to 9.0 compression if the rest of the parts match up. You also need to think about what octane rating gas is available there in France. I don't know but I suspect it's pretty low so you might not want a high compression engine. Put on an set of headers if you don't already have them. That's the best improvement for the price that you can make for that engine.

Have the ignition timing checked and see if you can get it up to around 16 to 18 degrees initial advance, that's after the other things are done. Big block Chevy engines like a lot of advance in the timing. You may also want to consider a little smaller carb. A 750 or 770 CFM carb with vacuum secondaries would work well on that engine. The 850 will work but it's more than your engine needs. Unless you have a ready supply of cheap gas, I doubt it, a smaller carb will work fine. Bigger isn't always better.

Not sure how easy it is to get all those things done in France. Here it's no problem at all. If you can't find an engine shop that knows something about Chevy big block engines, the crate engine as mentioned may be a better option since all the work will already be done. I know shipping and import taxes may be a lot. You have to weigh the 2 options, build your engine or buy a crate engine, and decide which is the best based on where you're located.