How much do you weigh? Size/weight will make a difference with fin choice. How long have you been sailing on the 147? Is this your first formula board?
First impressions on a formula board is that they want to go upwind more than any other board, but it doesn't take too long to learn how to bear off on a reach or downwind run. Mast position probably doesn't have much to do with your upwind problem unless you have it all the way back. A center location in the track is good. Slightly forward if overpowered and the nose is lifting and a little back of center for lighter winds and bigger sails.
I raced a 147 for a couple of years and never used anything smaller than a 68cm fin. There were times when a 65 would have been nice to have, but I only had a 70 Deb R13m and a Curtis 68. I also raced in winds over 25-28 knots a few times with sails as small a 6.6 and 7.6.
I am only an amateur formula racer and weight 77kg, but I could was able to manage the board in almost all conditions, with a small enough sail. My best GPS speed on a formula board was on the 147 with a 9.2 sail in flat water with the 70cm fin - 28.3 knots.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.