I'm old enough to remember when the first Pacers were released. The colour scheme that sticks in my mind was a white one with the red stripes running along the sides and over the boot, but that might have been a VG colour scheme. The Pacer was also the first local car with "tombstone" buckets - it was quite a ****-hot deal for its time.
Regarding the 340 - well, to be honest, being a Holden man, I have never followed the progress of the Mopar line development but I recall that, when new, the E55 Charger, with the 340, was a honker, considering it was just a stocker with a four barrel Carter (I think). The 360 that Mace mentions was only a 2 barrel and in standard form back then, was a real slug for its size, but obviously, the more cubes the merrier if you can find one.
Still, the 318 has the basis and the US aftermarket range for them is huge, so whatever you choose should make it a neat unit and something different.
Just make sure the front end stays stuck to the firewall and there is no rust behind the steering box. I can remember MANY Valiants separating at the firewall in prangs so it must have been a potential weak spot.