Does it matter which cassette I get for downtube friction shifters?
Unfortunately, your 1974 Campagnolo 5-speed hubs are likely to be 120mm spaced - you can see them in the 1974 Campagnolo catalog.  You're going to have a difficult time finding even old road hubs to fit that spacing.

I'm extremely doubtful that you have a 1974 Raleigh International in aluminum, since they were originally built with Reynolds 531 steel and Raleigh didn't list any aluminum frames in their 1974 catalog.  A quick test with a magnet should confirm for you.

Fundamentally, you're left with a limited number of options for the rear wheel, most of which have been hashed out above:


Continue using the current tubular wheelset. Simple, but you'll have to get more practice working with tubular tires.
Re-space the rear triangle. It's steel so, with caution, it should be straightforward. You'd need to add 6mm just to get to old 6-7spd hubs, and 10mm to get to a modern road cassette hub. If you're clever with moving spacers and wheel dishing you might be able to eke out another couple mm in there. A big disadvantage here is that you'd need to re-space the frame back if you wish to restore it to the stock hubs.
Find a NOS, used, or expensive Phil Wood 120mm freewheel hub and build it up with a clincher rim. The only limitation there is that of eBay and the fact that 90% of 120mm hubs you'll come across are modern single-speed track hubs, unsuited to your intended goal. Also a disadvantage, you'll still only get up to 6, maybe 7 gears total.  That said, building it up around a very similar wheelset to the one on there now except for clinchers is probably a good option - you might pay a bit of a premium for identical Campagnolo hubs of the same vintage, but you'll keep the aesthetics.