Yes it is possible, and yes it is do-able. I know of several people who have done this. There is a guy on the chevytalk forum who put a LT1 from an F-body in his 81-87 C-10. There are a few on sportruck.com who have converted. Typically the most expensive part is going to be the motor/trans/ECM. Being a new model it will be expensive. I haven't checked, but you might can pick it up at a salvage yard for around $5000. I know where you can get LT1/4L60E's with ECM for $5000 all day long, and he is high on his pricing.
As far as dropping it in, it would be pretty much a cake walk if you have done alot of swapping. The hardest part would be routing the ECM wiring and mounting. If you used a your transmission instead of the Electronic version 4L60E you would simplify some problems, especially since yours came with a 700R4.
Which would be cheaper? Of course building a motor would be cheaper. Did I leave some steps out, sure, It would take a book or at least doing it to cover everthing.
As far as weighing more, not sure how much the new trucks weigh, but my '73 non-power C-10 SWB with 454 weighed 3948lbs. A buddies ( http://www.shelby.net/mgervin
) '85 C-10 SWB weighed 3960lb. Here is a weight discussion we had a few weeks back: http://www.chevytalk.com/ubb/Forum45/HTML/000169.html