Hi Peter,
I don't know that particular cam from personal experience, but it sounds a lot like some other "3/4 Race" cams offered. It's probably a new cam rather than a regrind, which is always nicer. I thought the Moss "improved" cams were about 260 degree duration, but the figures you got might be more accurate. If you don't have it, you might ask them to fax you a cam spec sheet, so that you'll have all the timing info for setup (figures at .050 lift).
Similar cams are the Triumph/Kastner "D" cam, Revington Fast Road cam and #149 cam (Isky, I think). I've got one of the latter in a regrind that's 282 duration, inlet 31/71, exhaust 71/31, .431" lift at the valve (yours would be .435" assuming 1.5:1 rocker ratio, although if using the stock rockers you are more likely getting 1.43-1.44:1 ratio.)
Moss says .013" valve clearance hot. Cold, the #149 is .016" inlet and .018" exhaust.... probably works out about the same.
I think you will find the power band of that cam runs from about 1800 or 2000-5500. It will likely need a slightly higher idle setting, maybe 1000 rpm, and will probably be a little rougher idling than the original cam, but not bad.
In addition to the changes you have planned - porting and the headers are good ideas - you will likely need to change or modify the needles in the SU carbs a little richer. By the way, TR4A head should need minimal porting work.
Slightly higher compression would work well with that cam too, if you wished. Maybe .030-.090" off the head depending upon how much you want to increase it and what head gasket you use.
The cam should also work pretty well with 87mm pistons and liners, or even 89mm, if you wished. With 89mm a 280 duration cam is sort of like a 270 cam with 87mm. For similar effect a stock 86mm bore might use a 260-265 degree cam, approx. For comparison, the original milder cam would be around 240-245 degrees, if I recall correctly.
Other performance mods that would work well with that cam include a slightly larger diameter exhaust system, to 2.0, 2.125 or even 2.250".
Do watch that the cooling system is up to spec, since the various mods will make the engine run a bit hotter, but that sort of depends on how spirited your driving is, of course. Tube headers will raise underhood temps a bit, compared to the original cast manifold, too.
Another important thing to ask Moss is if their cams are hardened. If not, it would be wise to have it hardened before running it for the first time. You are probably aware that a lot of good cam lube during installation is important, too. And, finally, with that cam there is unlikely to be any problem but it's always a good idea when increasing lift to check that valve springs are in no danger of binding.
Have fun!
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