as the others have already said, put coil springs under a Series and it becomes a modified motor, quite heavily modified
now if you don't tell your insurance that it's been modified in this way you'll still get cheap(er ish) classic insurance BUT if you have an accident that requires your motor to be checked over you may find yourself in deep sh*t
and if that should happen, just try getting insurance again, you'll be paying through the nose for a very loooooooooong time
not to mention the possibility of court cases and possible results there
if you want something that LOOKS like a Series but more (?) comfort there's the easy way of buying a 90/110 as they're a very similar silhouette with as far as i can make out not much more in the cost of insurance
then are you looking to avoid paying VED ??, that again counts out 90/110 and legally counts out a coilsprung "historic" (pre 73) Series on the major modification to the chassis and axle/suspension swaps (again the risk of court cases, defrauding HMG is particularly frowned upon)
it's all a minefield with one of the mines waiting for you to step on it
anyway if you want a better chance of comfort in a Series without testing any legal limits i suggest you get a 109, i have both an 88" and a 109, my 88" hasn't turned a wheel in just about a year now as i enjoy driving my 109 so much (both have been my dailies over the last 5 years), the 109 is a far comfier vehicle with more space to chuck junk in, more space for fat and/or tall drivers (i'm skinny and small
) and so far no less capable when laning in Wales
i hope that's given you a few options to consider