going to a big block will be more involved, you'll need to either get conversion mounts, or a different K frame. you'll also need a different radiator and tranny, not to mention all the little things like accessory brackets (alternator, PS, AC, etc).

if you're on a really tight budget, I'd also suggest what I have in my car: stock long block 360, with a comp XE262, eddie 600 carb, eddie RPM air gap, and headers. dulcich did an article on an engine essentially the same as this about 7 years ago in Mopar Muscle and it made 335hp. this combo pulls with authority in my 3900lb 5th ave, which I've added 3.55 gears and an A500 tranny. butt dyno it feels about as strong as a stock hemi 300C, which should put it in the low 14's, maybe high 13's.

you should be able to find a good running 360 for ~$500, and add the cam, intake, carb, and headers for another $400-800 (the lower end if you can get say the carb and intake used)

another option would be to do the same but with a stock long block 360 magnum out of a '93-01 truck, pick up an eddie air gap for mag heads, swap the valve springs with
gm 3100 springs and retainers, and get the roller cam reground to about .305" lobe lift and ~218/222@.050 intake/exhaust duration. use the hughes cam snout extender & t-chain cover off of your 318 and you can run a mechanical fuel pump.

I'd think you could find a good running 360 mag for under $800.

you can reuse your small block tranny and converter, you'll just need to get the appropriate weighted flexplate from B&M to use either an LA or mag 360