putting a red top in that white GT would be criminal... Why do you NEED more power speed limit is 70mph ??
The future of modified cars is looking a bit shaky these days, BUT if your set in your ways and want to fit the red top series of engines, then it will fit, but its not a quick drop in ready made parts off the shelf (not like your old haunts).
Your going to need to uprate the brakes (no after-market direct fit items exist so its make it yourself), Lots of info on internet about home made brake conversions,
that may be deemed illegal soon.You will need to make your own sump,(possibly remove all the balance shaft part from the engine), Manta 1.8 sumps are rear these days but would make a good basis for the conversion.
You will need to move the ignition module out of the way of the bulkhead,(unless you commit sin and cut the bulkhead away)
UNWISE on many levels.
You will need to find a RWD gearbox (Omega manual box) two types one big one small big one will not fit unless you remove tunnel in the shell (UNWISE see link above)
The list is almost endless, 99% of it will have to be custom made, and frankly its been done to death!
One or two folks here have done the conversion I am sure they will pop in and say what needs to be said about the job, I assume you know one end of a spanner from the other?
As for the GT well with only
25 / 30 left on the road and maybe half that amount in garages it does seem the wrong car to undertake this type of conversion to.
There are other paths to updating a GT if your heart is set up this, The engine could be swapped out for the larger 2.3 unit and this has potential for a reliable 160 bhp and more if you are happy to splash the cash, which isn't so different to the red top performance, albeit its a slow lazy engine not a screamer like the red top. maybe look towards the
Lotus / Jensen Healy route as their engine is similar in design to the Vauxhall unit and again if its just pure get up and go you NEED then its another idea.
Anyway that my take on it, if you want to hash it about do it its your cash, but you will devalue it to about half what you paid, OR look at the 2.3 unit and some of the things that can do.