You will need a 454 flexplate (flywheel). The 454 is externally balanced. The 350 is not. It is internally balanced, so do not attempt to interchange the flywheel/flexplate between these engines.
How complete is your 454? The alternator, and PS pump use different brackets/braces. You can use your 350 water pump pulley, and crankshaft pulley, but the 454 pulleys are a different part number just for reference's sake. If you have a 454 crankshaft pulley you should use it because of the way it centers on the harmonic balancer. This is assuming that both engines are using the later model 1968~1987 "long" water pump, and before the 1988+ reverse rotation water pump.
Also, the engine mount "clam shells" (all steel engine side mount) off of the 350 will work with the 454. However, again, the 454 uses a different part number, and there is a slight difference in the parts. You should use the 454 parts here as well if you have them.
You are also going to want the 454's 7 blade clutch fan, and 4-row radiator as well.
I have done a ton of SB to BB swaps in 73~87 Chevy trucks, and it is pretty simple, almost a perfect drop in installation. But having done a few I have realized that you are better off using the BB specific parts even when the SB parts will work.
I can post all of the 454 component's OE GM part numbers if you would like, most parts are still available from GM. Good luck with your swap.