The differences between LS1, LS3 and LS5 are discussed on my earlier link. Just because a speaker was originally designed in response to a request from an OB team, or a studio team, it doesn't stop the design from being used in the other application - so yes, it wasn't at all unusual to see LS3/5As in studios, and LS5/8s in larger OB vans. BTW, one of my pet peeves - happily not mentioned in this thread: the idea that "LS3" = grade 2 and "LS5" = grade 1. The BBC never had grade 1 and grade 2 monitor speakers (though picture monitors could be grade 1, 2 or 3 - I guess that's where that could have come from).
An earlier design in a trapezoidal cabinet was the LS3/4. It was a long-standing model - some were still in use in TVC before it was sold. There were several variations along the way, including a very late one containing the drive units from the LS5/9 (designated LS5/11). There was also the LS5/2.
The LS5/8 versions (LS5/8L and LS5/8R) are mentioned on my site, with pictures:
https://www.markhennessy.co.uk/roger...m#ob-variation
The change of shape can only help...
The biggest problem with a regular box with 3 pairs of parallel sides is standing waves. Even with air damping - usually Rockwool or similar for BBC designs, but sometimes foam - you'll still get standing waves that could affect the measured frequency response. Irregular-shaped cabinets fix this problem, which is why curvy injection-moulded plastic and die-cast aluminium cabinets are so popular today.
Changing the panel sizes will change the frequencies at which they resonate, but as BBC designs use added mass to push these down in frequency (and Q), I doubt it causes any problems. If anything, it's likely to help, as it'll spread it all around.
The OB version I show on my site was probably the best-sounding pair of LS5/8s I've heard. They were assisted by additional EQ - stock LS5/8s are far from flat, especially at the bass end. I'd love to learn more about the response curve used.
The amplifier in the Spendor BC1A could suffer from thermal runaway. I know that a lot in BH were converted to standard passive operation (by replacing the mains inlet LNE with a standard XLR-3). We've still got a handful here at WN in original working condition, but they don't get a lot of use. Nice to have access to a classic though.
My experience with LS3/5A pair matching for stereo isn't so good. If they were the later 11 ohm model, you should be fine, but the 15 ohm models were all over the place. If you found an unmolested pair from the same manufacturer with similar-ish serial numbers, you might be OK, but no promises! I never managed to get half-way sane results with a pair made from an 11 ohm and 15 ohm examples, even though everyone had been told that was possible. The problems with 15 ohm models have been discussed here recently, and there's a lot about them on my site - it was hard enough when new, but they suffer terribly from ageing, and any surviving original 15 ohm model will be out of spec today, even if it has been treated with kid gloves, because of the glues and "dope" failing. This post has a bit more info:
https://www.vintage-radio.net/forum/...5&postcount=97
Wherever I've worked, I've kept records of the LS3/5As in service, and swapped them around to match operational pairs as best as possible. It usually helps, but not always - especially if they've been worked on in the past.
The LS5/9 was never popular, but it commits no more sins than any other BBC design - it's just unfortunate that commits the wrong sorts of sins. It's also very prone to ageing. However, in my direct experience, everyone changed their opinion on the LS5/9 when they heard them properly - several people I know bought pairs for themselves after a visit to my place. They're quite fussy about stands and placement. Get that right - which we never did in the BBC - and they are surprisingly good.