Pondering a 305 rebuild vs. LSx Swap

[KMK454]

I bought my 78 about 5 years ago with 18k miles on it. I say it's all original but the reality is it's had some replacement parts here and there, some of the paint was poorly redone by the first owner, the emissions equipment is missing, and the exhaust from manifold back is not factory. Currently it has about 36k miles on it and I drive it almost every day.

As I've been replacing and fixing things on it to keep it roadworthy, I have yet to do any intense engine work aside from seals/plugs/wires/fluids and a carb rebuild. It runs very well but will need a rebuild sooner than later to run just right. But instead of putting all that effort into rebuilding the 305, I was pondering doing the following:

LSx (either an LS2 or LS3)
4L60E (would prefer manual but do not want to cut floorboard)
Any other parts needed to get the LSx setup running (wiring, fuel system, etc.)

The only exterior change would be widened factory rear wheels that would still sport hubcaps and whitewalls, so it would kind of be a "sleeper" except for the exhaust note.

Do LSx swaps require cutting or modification of the frame and/or body work? If I go through with this I want it to be as reversible as possible, with the stock 305 put in a crate.

The truck would lose that original look under the hood, but I predict it would get great gas mileage and the "character" that comes along with an old carb'd engine would go away. Thoughts?

Here's how it sits now: