I'm assuming it's the four door hardtop (Holiday Sedan). A friend of mine had a full sized one, a Dynamic 88 not the Super trim level as modeled by JoHan.
For the chassis, how accurate does it need to be? I seem to recall the kit is actually around 1:24 scale. Note that these cars had a perimeter chassis. JoHan promos had a simplified but fair rendition of this, while kits I've seen had a chassis carried over from 1960, which is a completely different design. Maybe you could look for a beatup X-El reissue of the '63 Starfire as a donor (show below).
Wheelbase is 123". IIRC the old AMT annual Pontiac Bonneville chassis might work. Chassis is similar in shape between Pontiac and Olds, but slightly different. Suspension components are different in shape between the two. The trunk floor and rear frame under it are different between the two as well. The Olds trunk has a deep well, below the frame. Its gas tank is transverse behind the axle. Pontiac's trunk has the gas tank mounted flat under it. The main floor pan itself appears to be shared between the two. The interior will need a lot of work to get together, as most kits of the era we have handy will be two-door hardtops or convertibles. The dash will also be a challenge if not included with the resin kit. The original JoHan interior and dash were a little shallow and undersized.