TH200 4r Lock-up

My 39 Olds has an Olds 350 engine with a th200 4r trans. I'm running the original 39 Olds rear end which has a 4.30 gear. The transmission is stock, with no performance modifications. I did replace the stator support shaft and installed a rebuilt pump along with new seals, filter and fluid. Although this trans is from a salvage yard and I don't know the mileage on it, it seems to work fine and shifts firmly enough.

My question is about the lock up point. The converter, as it's now wired, locks as soon as the trans shifts to second gear. This seems to make the 2-3 and the 3-4 shift rather harsh or rough. I'd like it to be smoother. Also, because of the low rear end ratio, I believe, the trans doesn't downshift to first gear until the car falls below about 5 MPH, and the converter doesn't unlock. I have the lock up wired through a relay connected to the brake light switch, which unlocks it when the brakes are applied, but when I lift up on the pedal, it locks up again.

My question is this:
My Son-in Law tells me there is an external pressure plug on the trans that can be tapped into which has pressure only when the trans is in 4th gear. He says a pressure switch can be installed there that can be used to apply the lock up only in 4th gear. This seems to be what would take care of my harsh shifting. If this is true, where is this plug? Will it work?

Yes, I know there is a kit I can buy that will do what I want, but if it's possible to do it another way, thats what I'd like. Thanks for your help.

There should be a 4th gear pressure switch in the pan on the valve body, There are 3 different versions of this switch, 2 terminal. Normally open and Norm closed. The easiest is the Norm Open. Power wire into the trans to the lockup soleniod, Ground wire from the lockup sol to the pressure switch. This will cause lockup in 4th only

I have a 700R4 Mated to a 350, and the brake light on the dash board keeps coming on, now I don't know if the Lock-Up on the TC is connected to the brake switch, but sometimes when I push the peddle down while I am driving the light goes off. How do I know when the TC if locked or not, when do you want it to lock up, i.e., in 2nd, 3rd, 4th, all gears except 1st. Will having the TC lock up or unlock cause any problems with leaks in a Transmission? Would it cause the tranny to over heat? What would cause seals to start leaking? When I drive the vehicle the tranny seems to heat up, if I pour water over the tranny after a drive the water steams when it hits the tranny, I think the tranny is over 200degrees when running, the tranny cooler lines run into the radiator, how critical is an external cooler? Why would the tranny want to Kick in and out of lock-up when in OD or 4th and lightly pushing on the gas or even heavy gas, you can really only feather the gas peddle to keep it in 4th...

I was hoping to do something without dropping the pan, but guess I'll have to. Thanks for the help.
Bill