You're talking quite a difference between a ZZ-4 and a used 302. I have a ZZ-4 with a GM hot cam and programable fuel injection. Long tubes, Kevlar clutch, T-56. By the time I was done I spent $12,000 USD! JUST ON PARTS. I'll sell you the whole car for $10,000. It rips. Dynoed 410hp. Has only 25,000 miles on the swap. 13.25 @ 109.9 1/4 mile.
Last week I worked on a 94 T-bird with a 4.6. I was impressed with the torque and tire smoking capabilities. There are lots of police cruisers on the road with this package that can be had cheap...very good possibility. Any 94 Caprice police package has the LT1 engine this is a nice swap and very fuel friendly. 26 mpg cruising. And the lack of a rear ignition distributor make the swap very nice so you can mount the engine low and rearward for great handling.
Another tip. Choose a Volvo that isn't all beat up. Mine was a well used 740 that needed interior work and paint. You will spend so many hours installing and perfecting your swap that to choose a vehicle that needs lots of attention anyway just isn't fair to yourself. If it's worn out SHIP IT OUT! Find a nice Volvo to start with as these cars are keepers once they are done. After driving a high powered Volvo it's hard to get back into a weedwacker again. They handle nicely too.
Another tip. If you do decide to buy a conversion that's already finished get it checked out carefully. Some guys that are so called mechanics shouldn't be allowed to fix cars. Have a garage that knows Volvos check it out for you so you are assured it's safe and not prone to fires or a driveshaft improperly engineered.
Good Luck