Can't help with positioning but

yes a fin box will work but a proper mast track box costs the same. Or get an old stuffed board for free and cut it out.

Set a block of divinycell about 2" bigger than the box (in all dimensions) into the board with epoxy and q-cell (a bit lower then the deck) and then set the box in that, then bevel glass edges on the board so your glass job feathers in and glass over with 3 layers.

have a look a boardlady.com

I am a backyard board builder. Currently enjoying a sup when conditions are small.
This board I think would work as a strapless light wind s'board.
it may not as some sups are surfboard construction so are built light and don't handle jumps.it depends on the board
Is there a dimension for locating the mast track?
yes 1250mm to back of track
Is a mast track just a fin box?
yes, with some hardcore foam for strength.
Will a 5.5 sail get 75Kgs on the plane with a 110Ltr board.
yes, but board width and wind strength is a factor
The Kona Sail Mals appear to surf well in the photo's is that correct.
yep, kona rock! guy at my local spot has a 10'5 and 9'5 and does back loops, air gybes, rail rides, 360's, carve gybes, tacks, and beats the freeride crew both up and down wind. he uses a 5.8m wave sail. that's all on flat water.

i haven't seen him in the surf but he rates it for that also.