Glad to be of help. This does not upset the brake balance. The only question is whether or not there is enough room to fit the booster. You are correct in assuming it only increases the brake force. There is no reason you can't add one if space and availability is possible, and apparently there is since you can get one here: http://www.theautopartsshop.com/auto+car-usa+parts+year/triumph+spitfire+power-brake-booster+1979.html
I'm switching to Q&A format if you have more questions.
The brake balance is controlled by the proportioning valve. The booster has nothing to do with it. If you have front disc/rear drum now you already should have this valve installed. If you are converting to disc, you will need this valve.
Let me know if you need more help.
I feel I could use a combined master/booster, like an MGB which is first link in system and ahead of the valve. Therefore would not have to use two remote units. Since the bore dia is similar this should work and cut down on piping. Thank you
That is the only way you can use it. Perhaps I misunderstood you. You cannot use two master cylinders. The dual port master has two separate sections, one for front brakes and the other for rear brakes, separated by the proportioning valve after master cylinder. The booster is behind master mounted to firewall with master bolted to front of booster. These are two separate units but work together. You can buy them new or rebuilt separately or you can buy them rebuilt together.