Blowing a 30-amp fuse in succession indicates a direct short to ground somewhere. Check all the wiring to make sure it's not grounding out anywhere. The fuse is for the protection of all the wiring but the main circuit is the operating circuit. Kick down is a separate circuit but is part of the complete circuit for operation in all modes.
The coil doesn't need a 10-AWG wire to ground it out when you hit the kick down switch. A 16-AWG would suffice. The solenoid operating circuit can get by with smaller wire on 12-volts. Does this transmission still have a rail cut out switch or has it been removed? My early 51 cars still have them but they have to be maintained or bypassed. Does it blow the fuse when in overdrive or just when you try to kick it out of overdrive? There is a point set in the solenoid that controls the coil grounding function in kick down and there is a point set that controls the switch from the overdrive pull in coil to the overdrive hold in coil function. The pull in function is a high amp function so it needs to go into holding function as soon as it's in overdrive so as not to pull too many amps. The kick down switch is in a swampy area so it has to be maintained in good condition as well. The governor is there to keep the unit from going into overdrive below 25 MPH or so. They can get a build up of crud in the points now and then but the overdrive usually just doesn't function if the switch isn't working right.
Make sure the operating solenoid is properly connected too. It can sometimes be something simple like that. We all make mistakes now and then. I know I have. A Link:
https://fifthaveinternetgarage.blogs...ve-wiring.html