'68 Valiant power steering gearbox leaking-Seal replacement?

It's not that hard to replace the seal on the pitman arm, but you probably need to look at all of the seals in the box. Autozone sells em rebuilt for I think last time I checked $129. Thats not bad as long as its a good unit.
It would be up to your mechanic to tell you how much HE would charge, but my guess is he will charge quite a bit. Thats why when you have older cars like ours it really pays to do the work yourself.
I don't know how much he charged you to put on the brake cylinders :evil: but I know I just bought them for 9.99, and payed zero to have them installed. :D :!:

Part of the fun is getting in there and doing for yourself. You learn as you go and get a great sense of accomplishment as well. Unless this is your daily driver and absoloutly need this car fixed, grab a wrench and dig in!