So I just put a 302 engine from a 1968 mustang I found in my 1974 bronco. Ran great for a week then started stumbling at low rpm. Dies at idle. Missing and getting worse. Seems to be exacerbated by heating up. First I thought it was older edelbrock 600cfm I was running. Got a new one no help. Since then I have replaced fuel filters coil distributor plugs wires. Still doing same thing. Beginning to think it might be electrical. Wire from distributor goes to - side of coil and hot wire goes to + side of coil. With key switch on the hot wire only reads 6 volts. Is this right? Any other suggestions? Other than exorcism?

 

Have you checked the points gap?
Make sure the contact heel that rides on the cam isn't worn out.

 

Have you checked the choke? Electric or heat tube? Any loss of antifreeze, or sweet smell in the exhaust? Have you checked the valve clearances? 6 volts is about correct at the + terminal on the coil. With 12V, the coil would over heat so make sure you still have the red wire or a proper resistor in the coil lead if you are using a stock coil.
Other factors, electric fuel pump? Proper adjustment of the floats and needles on in the carb.

 

Rather than jumping around, I would first exhaust each system by process of elimination. Check the + coil wire when hot and see if the voltage degrades with temperature. You should have 9+ volts so it could be the primary wire running from the plug at the firewall to the coil.

Good advice to check the points and cond. Again, once you've eliminated a system, you can move to the next such as fuel, which doesn't appear to be heat related.