[MagicRat]

Which car?

The first thing is to determine the GVWR (Gross Vehicle Weight Rating) of the vehicle.
Often this is placed on the vehicle, usually on a door jamb or glovebox door, and/or printed in the owner's manual. Typically, it is given for each axle, and a combined total is given.
Often, a recommended tire size is included in this info.

Now, I ask 'which car' because I bet this is for the limo, right? And, since very few were made, I bet there is no GVWR sticker, right?
Well, this makes a difference. You will have to estimate the GVWR.

1. Take the car to a weight scale. Often, places that sell sand, gravel, topsoil, asphalt, landscaping materials etc have a scale you can drive on.

Sometimes, auto wrecking yards have scales, or there may be a DOT truck inspection facility with a scale.
Ask the sale operator nicely if you could weigh your car.

2. Now, calculate the max load that car could carry. Is it a 9 passenger? Okay, allow 200 lbs per person plus 300 lbs of luggage. This gives 2100 lbs.

So, lets say the empty car weight is 6000 lbs. Add 2100 lbs and you get a GVWR of 8,100 lbs.
Given the car's length its reasonalble to assume that the weight, when loaded will be evenly distributed among the 4 tires. So you want tires with a maximum load of 2025 lbs per tire.
This is too much weight for any passenger car tire. These are prefixed with a 'P', like "P235/75R15". You will need a 'Light Truck' tire, which has the prefix 'LT'.

This max load rating is stamped on the sidewall of every tire. You must fill the tire up to the max pressure shown if you expect the tire to hold this weight. If the load is less, you don't need to add quite so much air.

Next... tire size. I am certain LT235/75R15 will fit on your car. This is an extremely common tire size. Don't get an agressive off-road-type tread pattern. Get a mild, all-season tread, for a quieter ride.

My Uniroyal Laredo tires, LT235/75R15 are rated at 1975 lbs per tire, which is typical for a 4-ply truck tire. This may be close enough for you. But, one can get 6 or 8-ply tires in this size which can hold more weight so long as you add more air pressure. But they will have a harsher ride.

Note. My passenger car tires in this size (P235/75R15) can only hold about 1600 lbs each - not enough for your car, imo.

As for the aftermarket wheels.... ask the dealer or manufacturer. They should have specs. IMO the Catalina will be too heavy for aftermarket passenger car rims and you will have to look at light truck rims.... which are made in the 5x5 (I think) bolt pattern your car probably has.