Both will fit, although slight massaging of the
tranny tunnel will be necessary with that C6. It's a huge
tranny but will go in. The 351 can go with no problem at all. You just change the mounts to V8 mounts, and then get a 351 swap kit. It'll include an oil pan, headers, accessory brackets, and a few other things to make it a drop-in. Then you add a 351 distributor, intake system and fuel system capable of feeding it and you'll be set. It sounds like a lot but it isn't too hard. The only real snag will be to make sure the headers are compatible with the C6.