New car have much more rigid shells, the cossie shell was not motorsport to start with, just a shopping trolley car 20+ years ago, a lot of this has come about due to crash test ratings.
Newer cars also have vastly better MATCHED spring and damper rates compared with cossie days.
There is nothing to stop you doing what you want, but won't work well.
The fundamental problem with the sierra is it is a bit shit really, well totally shit as std for a performance car in the handling/road holding modes.
Most of this is due to the diabolical rear suspension, which is fine on a car driven by a old granny to pop out to bingo etc, but not spirited driving.
Ford for some reason must have used different design teams to do the front and rear suspension, because very clearly they never communicated with each other, just like the car doesn't, there is a monumental stiffness difference front to rear.
Just adding a poly diff mount £90 on ebay will foo, you into thinking you're in a different car, the difference really is that vast, and this removes a good 80% of the roll, as it stops the beam jumping around as though on e's at a disco!!
Add to this a 22mm rear roll bar, and you will be overjoyed with the difference, even if on std arm bushes.
I ran with NO rear arb for 3 months after fitting a poly diff mount, and noticed around 80% LESS roll than a std diff mount with arb(16mm), then add a 22mm arb to this mix and it is so much better again, mine is adjustable as is custom made out of EN16T correct sprung steel to completely clear the diff mount and tow bar bracket.
IN NO WAY DOES IT MAKE THE CAR TWITCHY, it gives vastly better feel that you know exactly what the rear is doing, I didn't listen to the experts who told me it wouldn't work, as I like to do what I want and find out for myself, that's why my one is adjustable, so it can be made softer or stiffer as needed, present setting is mid way/ish same as a escos arb.
The whole car is so much more composed, but bang per buck difference is without doubt from the poly diff mount, stops all the inherent rear wheel steering when hitting bumps mid corner, so do this first, don't touch the front at all, it seriously doesn't need it, the rear is the problem, now run 22mm arb and poly diff for over 2 years, a lot nearer to what it should have been to start with imo, the mere elimination of the rear steer upon bump/rebound puts it in another league.
For those wanting "purest" looks for bufties etc the std diff mount can be filled with poly so no fitting probs with arb, and the cost is MUCH cheaper at around £35, have had this done for a few others that asked and they've been very impressed.
tabetha