That's a real shame (& a BIG red flag) that some dope replaced the Olds engine with a Chevy. The fact that none of the gauges & speedometer work suggests to me that somebody did this conversion on the cheap. If it were easy to get all this stuff working, they probably would have done it before putting the car on the market.
There are a lot of pitfalls lurking in an engine swap of this nature. This is not a drop-in to do on a weekend over a few cases of Bud Light. Others here are likely more savvy with this sort of thing than I, but you're likely looking at a different engine mount setup, a different or relocated crossmember, & a new or altered driveshaft.
Those are some of the bigger issues. You've already described some of the smaller ones. Here's another: The Jetaway Hydramatic used on the older Olds had a gearshift indicator sequence of PNDSLR, not the standard PRNDSL that would be on the current transmission. If nobody bothered going through the trouble of hooking up the speedometer, I'd be astounded if somebody spent the cash to fix a little detail like this.
Come to think of it, the speedometer could be wildly inaccurate once you figured out how to connect it, because the speedometer drive gear in the trans very likely doesn't match whatever rear end is in the car.
I don't want to be a crepe-hanger, but I'd be very suspicious of the workmanship that went into swapping in a Chevy motor. Even under the best of circumstances, this is an ill-advised conversion for this vehicle.

Clearly it was not done by someone with a knowledge & love of vintage Oldsmobiles. Unless that person happened to drop many tens of thousands of dollars turning this car into a Foose or Coddington-inspired rod, a well-restored convertible of this era will almost always have a higher value & greater potential for appreciation than a Chevy conversion, especially one so half-baked that the gauges & speedo aren't hooked-up. I can only imagine what else may be wrong.
Tread VERY carefully. Have you seen this car in person? Is it possible for you to post a few pics of the car? Are you planning to buy it from one of those classic car dealers? If so, given that you're out of the country, the guy must be foaming at the mouth over this.
If you haven't already done so, it would be well-worth your money to have the car inspected by a disinterested third party. I've used this outfit before:
http://www.i-van.org/
It was the best $150 I ever spent. These guys are all AACA judges & have a keen eye for trouble spots.
Without seeing pics of the car, what I'm about to say is based on a little bit of fact & a lot of conjecture: If you are truly unfamiliar with these cars, you probably would be better off waiting for a well-restored car, or even a tired, relatively unmolested driver. The '60 Dynamic 88 convertible is not particularly rare- Olds built 12271 of them, & there are decent ones to be had if you're patient. You will come out far ahead in terms of money, time, & aggravation. If it's already a done-deal, I hope you stole it for a terrific price- I can't imagine the cost of shipping a car to New Zealand.
I hope I didn't ruffle any feathers, but over the years I've seen so many nice folks get burned with cars- I'd hate for it to happen to you!
Chuck