Well my first choice would be Chevrolet. General Motor Performance Parts Division (GMPP) will build you a brand new Chevy 350. It comes with a one year from date of purchase GM factory warranty, such that if anything goes wrong with it in that time frame you can have it towed to any GM dealership and they will fix or replace the motor for free just as if it were a new car.
Of course you will have to clean the shoe polish off the windows and remove the slicks off the back first (so don't tow it directly from the race track unless it is still all stock looking). And the down side is it is a stone stock base Chevy 350 motor so it won't win a lot of races in a full size Impala anyway.
You can buy this motor for less than the cost of a pile of parts that I would have had to buy to build a stock Chevy 350 (back when I was still doing that sort of thing). So the price is right, and the warranty can not be beaten with a stick. You can up the antee and also buy a brand new GMPP 700R4 transmission bolted to your SBC 350 ready to drop into your car. It also shares a factory one year 50,000 mile warranty.
Now GMPP sells a large number of small and big block Chevy engines and parts in their on line catalog. You can down load the whole thing for free or just look on line and print the pages that interest you. Most never discover the Circle track race car engines that Chevy builds for claimer racing.
Once again this isn't a warranted motor (race only) and the fasteners don't come off easily as they are made for a claimer racing program that highly discourages any modification of the engine (every one gets the same motor and there is no cheating allowed
but this is dirt track racing so racers just get more creative at covering their tracks). I couldn't build one of these motors for less money than Chevy sells them as it is a quality build with all forged internals and with a decent hydraulic roller cam.
GM 19318604 CT400 IMCA-Sealed 604 Chevy Crate Engine