Any porter made before the 1800's would most definitely been 100% brown malt. However, brown malt of that day was 100% diastatic while todays malt is not.
Once brewers began using hydrometers they discovered that brown malt, while much less expensive than pale malt, was not as efficient. It was actually cheaper for them to use pale malt and so economics won out. They still used a considerable amount of brown malt however.
The earliest porter recipe I have in my collection is from London's big porter producer, Barclay Perkins. The recipe is scaled down to 5 gallons from the original hand written brewery logs. It comes from the book, Home Brewer?s Guide to Vintage Beers by Ron Pattinson. Page 42
And Ron describes this as a transitional recipe, between the 100 percent brown malt 18th century grists and the overwhelmingly pale malt ones from the 19th century. Since you would have to malt your own brown malt to make this a true 1700's porter this is your best bet to get as close as possible.
1804 Barclay Perkins TT
Batch Size 5.5 gal
Boil Time 90 mins
46.3% Brown Malt 6lbs 4oz
40.7% Pale Malt 5lbs 8oz
13% Amber Malt 1lb 12oz
2.5oz East Kent Golding (5% aa) Boil 90 minutes
2.5oz East Kent Golding (5% aa) Boil 60 minutes
British Ale Yeast (Wyeast Labs #1098)
Mash at 150 - 152 F
These figures are accurate for my brewhouse/mash efficiencies. You should scale the recipe to match your system which is why I gave you the percentage figures for the grains. You could probably scale the boil time down to 60 minutes but be sure to adjust the hop schedule as well.