Is there a kit of some kind that will allow me to stick a 4.3 v6 or small block chev. in my 1974 opel manta
Erm no! theres no "off the shelf" kit for fitting any engines to the manta A or B. Do some research sure someones probably done somrthing similar, The Rover V8's are a popular choice.
i agree Rover V8 is the best way, there are kits to match the engine to the original manta gearbox, if it's an auto box there may be problems with the torque converter