Easy bolt in swap. I think the clam shell motor mounts is about all that has to be done, and the crossmember should line up with a different set of holes already punched in the fame to accomodate the Th400. Though IMO, the 454 if it's got peanut port heads, isn't going to give you the gain you think it will. Personally, I've liked the attitude of my 350/Th350s better than the 454/Th400. IMO, the The OEM 350's just seem more spunky, like the higher rpm and make them more fun. Only time you'll notice any gain with an OEM Peanut Port headed 454 is if you're towing something. Now do a head swap and an RV cam and intake on the 454, total different ball game. You'll wake that bitch up.
Just saying, unless you put some adders on the 454, just don't have high expectations for a performance gain unless your 350 is totally wore out and done having fun.