I've always intended to rebuild and keep the original 318; I know 'all original' doesn't carry the value for a base model that it would for an R/T but still want to keep it. The 360 plan was (and largely still is) a "340 Six Pack" on the less expensive 360 short block; even 25 years ago when I started collecting parts, 340 blocks and cranks were stupid expensive and their rise in cost has more than kept up with my rise in salary, and extra 20 cubic inches... why not? I've got a 360 core engine that still turned freely, 308 heads, intake, carbs, air filter, most of the linkage and fuel parts.
The 360, due to the fuel system and more power, is a more complex beast. The 318 was still running decently well when the car was parked. I have a nearly new carb and a quality rebuild kit for it. It still has the 904 trans and the small pinion 2.76 8-3/4" axle; with the 360 I want an A518 overdrive unit which I don't have yet, and a large pinion pig in the rear axle (I have a 1-3/4" 3.55 setup ready to bolt in). So with the 318 I get a running car back sooner, even if its not completed to the level I eventually want. And again some practice doing a less expensive engine build (post machine-shop) before I tackle the the bigger one.
Of course who knows, in a few years I might just decide to drop a late gen hemi in it instead if I decide the six-pack is too much hassle... and if I can afford it. But it will have been running and usable with that little 318 in the interim.