Hello Samuel, A 7Ib cap will be ok, use a 50/50 mix of Blue antifreeze and see how you go. By the way, our flathead engines do tend to run a little hotter than most because of its design. Your temperature gauge takes its reading from the centre of the cylinder head (where the top hose connects). I mention this because this area is also shared by two common exhaust ports just below the head. As a result, the temperature in this area can get quite high and your gauge will read accordingly. Bear this in mind when looking at the temperature gauge. Modern cars take there temperature readings from the radiator which is more a average temperature rather than a hot spot like ours. Please keep in touch and let us know how you get on with things.
Kind regards, Charles Gough