As Wayno said, a four cylinder L, or LZ series will fit with a small amount of modifications....a thinner radiator, eliminating extra pulleys, etc.... I put an LZ20 and dogleg in mine, and it ran really nice. It is a tight fit though.
The Buick V-6 needs to be set back approximately 3 inches for starter/front crossmember clearance, and still takes up all avaialble room. I did the swap once, and would not do it again....just too much butchery. A 60 degree Chevy V6 might work a lot better, and be very peppy. If you insist on using a V8, and want to get into the 10's at the track, then you may as well spend the bucks building a fiberglass body and a full tube framed drag chassis, because the stock stuff just will not work. As to the inline 6 cylinder....it will fit in the bed, just make sure you block it so it will not fall over, and use good ratchet straps to hold it down.
That being said, mklotz and I have pondered putting a Toronado big block front wheel drive unit in the bed of a 520/521....much fabrication would be required....however....