I have a 97 gtb wagon manual i have put 99 gtb auto engine in and have modified inlet manifold 2 suit idle valve engine is running fine and goes hard bar a bit of detonation.been told that it is compression ratio difference any 1 know how i can decompress engine ? eg pistons head gasket .... any help appreciated or knowlege of how to overcome this issue

 

thicker headgaskets is easiest way

 

It may be that with the addition of Nitrous Formula NF Octane booster you can overcome the DET simply by running booster. Certainly for the cost of a few bottles it is worth a try.
You can start by running it at 3 mls per litre of petrol. You will have a 60 litre tank so start with 180 mls and from there on add 3 mls per litre of fuel added. Put the NF in before the fuel so it mixes.
If 3 mls per litre cures the problem, cut back to 2 mls per litre.
It may take a tank of fuel usage before you get a representative result.
Where ever possible use Shell V-Power and if not available use Super 97 Ron.