furnaces are sized by BTUs and efficiency, not by tonnage like AC.

what size AC do you have now and what size has been quoted?

IMO

A manual J is really nothing more than an educated guess. Just as easy to get it wrong.

FACT: An air conditioning system is over sized 90% of the time it runs. Regardless what methods you use to size it.

A manual J is only a tool for those who know how to use it properly. If you are going to reinsulate after the AC is installed the unit will be over sized above what it should be anyway.

Price variation between a furnace and the next size up is usually small. But stepping up 0.5 or 1 ton in an AC condenser size can be a significantly more depending on the brand and model. If you oversize the AC condenser, then are sometimes forced to oversize the furnace in order to get a blower larger enough to provide the sufficient air flow for the AC. You end up with a big furnace in a climate like Los Angeles.

You 1960s furnace is likely to be oversized and have a low efficiency rating. Contractors who don't do calculations tend to want to size the equipment to what the house already has. People give stellar reviews to HVAC contractors based on their service and price. They have no way of knowing the bought a piece of equipment that is not appropriate for their house and climate.

You should do you insulation upgrades first, then calculate the size of the equipment. If you are interested you can buy your own software for $49.

Thanks, everyone, for your input. We do not currently have AC, just a 45+ year old 5-ton furnace that's horribly inefficient and oversized. My husband is intent on DIYing the insulation. The existing ducting, which we are keeping, is just laying on the floor, not strapped to the roof rafters. Combine that with the dinosaur furnace, and we'd really prefer to insulate after the install, when the attic floor will be more exposed and accessible. We've told all the contractors this before getting their estimate, so it should have been factored into their magical, mythical "calculation."

Insulation looks rather easy by principle but you have a myriad of things in an attic that if done wrong will not measure up fully. Your house is a shell and the thermal barrier of that shell is what keeps radiant heat out. Heat is energy and attracted to cool... radiant heat doesn't flow from hot to hotter.

Insulation is a dirty, scratchy nasty job. You will be forced to put your body into positions best left for that of a horror movie. Toward the eaves of the house you will be challenged because there isn't much room and sometimes the eaves extend outward a good distance. If you mess this part up the value you are placing on the insulation project that hasn't been done will not live up to what you think it will.

The AC company will then get blamed for your handiwork when the AC can't keep up on the hottest day of the year.

Realize I've been to homes with thermal barrier problems of someone who thought reinsulating the attic was an 'easy' job.