I have a feeling height may be an issue, and perhaps getting an oil pan to fit. if you want to stick with 6 cylinders, I think I'd try adapt the 4L's EFI to the slanty, or look for a take out 3.7 from a liberty or GC, a magnum 3.9 (3/4 of a 318), or a 3.8 out of a grand cherokee.

personally, I'd build a 318 magnum similar to hotroddave's "super duper gas mileage 318" in the "best of" section, but using a reground hydraulic roller cam and either adapting the magnum EFI, or putting a carb intake and running a demonsizzler tuned small t-quad on it. or a FAST EZ-EFI.

I built a magnum headed 318 in my 4000 lb 5th ave, with a rerung short block, reground roller cam (259/259 adv, 208/208@.050, .506/.506 with magnum rockers, 112LSA, installed at a 108 ICL), headers, eddie air gap and edelbrock 600cfm carb, and it moved my old 5th ave pretty well, comparable to an early 1st gen 300C AWD from a roll. this was with 3.55's and an A500 OD tranny. in my mixed driving commute I'd get about 17.5mpg. now replace the eddie 600 with a spread bore or better tuned EFI, put it in a car 600-700lbs lighter, and switch to an A833OD with 3.23 gears, I bet you'd have a car well into the 13's that would return 20-21 mpg in mixed driving if you kept your foot out of it, and straight highway mileage close to 25mpg...step up to 0 deck with KB167 pistons, and expect more power and probably even better fuel economy.

my cam was reground by bulletcams.com, they used their HR259/316 lobe. step it down to their HR247/306 (247adv, 192@.050, 112@.2, .306 lobe lift, .490" valve lift with magnum 1.6 rockers) or their HR252/320 lobe (252 adv. 195@.050, 115@.2, .320 lobe lift, .512" with 1.6 rockers) and you might be able to get upper 20's on the highway....