First, the 273/318 had light duty rods, where the 340/360 came with heavy duty rods... Light duty will help save some weight, but you may have to get the crank rebalanced...
Also if you go with Keith Black pistons, they usually run lighter than factory pistons, but the skirts are thinner. I had one that broke on me so I'm cautious of them..,.
Speed Pro/ Sealed Power pistons have thicker skirts and weigh close to stock...
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You may want to look at some of Mancini Racing's rebuild kits, it may save you some $$$$...
Here's kit A & B for basic refresh with bearings and gaskets:
Kit A is basic:
Sealed Power Engine Kit - 340
Clevite 77 Engine Kit - 340
Kit B is the same but moly rings:
Sealed Power Engine Kit - 340
Clevite 77 Engine Kit - 340
Here's the kit C with bearings, oil pump, gaskets, cam bearings, freeze plugs, moly rings, and cast pistons. Make sure that they are the compression that you want, or call them for other options:
Sealed Power Engine Rebuild Kit - 340
Clevite 77 Engine Kit - 340
Main page for all small block rebuild kits:
Engine Rebuild Kits
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Here's a good timing chain that's one step above the stock 340 double roller, a true roller double roller. I've used these for many years and they hold up well...
Edelbrock
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I would recommend getting the heavy duty distributor drive gear from Mancini Racing as they are stronger than the stock ones:
Mopar Oil Pump Driveshaft & Distributor Gear
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The 273 adjustable rocker arms are good, but you will not be able to run the stock 273 push rods as they are made for a solid lifter which is different height.... If you want to run the stock 273 rocker arms with a hydraulic cam and lifters, then refer to these threads for how to get the proper push rods:
273 stock push rod length?
Pushrod for hydraulic cam and 273 rocker arms
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As far as timing cover and water pump, that should be determined by what year car you are going to put it in... If you are putting it in a 69 and earlier car, use the ones on either of the engines that you have... If you are putting it in a 70 and later, get the later timing cover and aluminum water pump so the radiator hoses match the ones in the car...