The spacers are really only nevessary if you want to use one of the better Cleveland intakes and you will find them here: AP-29 & AP-30 Intake Adapter Kits
A set of closed chamber 4v heads will bring your compression up quite significantly and the 400 has the cubes to for them to not be lazy heads in the bottom end. The problem you'll run into at this point is that doing the 4v head swap will be like buying 2 intake manifolds (and thus why I say cost prohibitive). The best bang for your buck will to get a fresh set of early 2v heads with stud mount roller rockers and larger valves, new cam and lifters, a performer (or if you can find one a Street master) intake, and a better ignition system.
I've got a decently built 400 in my ranchero (previously in my 72 Torino) and the thing just turns the tires to smoke when I goose it a little, when I was building the engine I never even imagined that it would have as much power as it does
Here is my build:
400 stock spec bottom end (yes, awful dished pistons)
'71 casting Cleveland 2v heads machined for 2.19"in/1.71"ex valves, 5 angle valve job, machined for 7/16" studs and guide plates, 1.73 ratio harland sharpe roller rockers.
Intake: Edlebrock Streetmaster (low rise single plane)
Generic RV cam: Camshaft - RV - Duration (I) 204 .050" (E) 214
Adv Dur (I) 270 (E) 280
Valve Lift (I) .484 (E) .510
Lobe Sep (I) 107 (E) 117
Pwr Range 1,500 - 4,000
Speed Demon 650 DP modified with annular boosters and 3 circuit metering blocks sitting on top of a 2" 4 hole tapered (knife edged actually) spacer.
Ignition: Mallory unilite dizzy (all mechanical) and holley promaster resin filled coil 55,000 volt output
Exhaust: 351c 2v headers to 3" H pipe