Are you saying you can only boil 1.5 gallons? How big is that pot, 2 gallons?
If the pot is 2 gallons that leaves you with 2 quarts of headspace. That's kinda tight, so you got to watch like a hawk for boilovers. Have a spray bottle filled with water handy, to spray the surface when it starts to foam or boil over. Also reduce the heat source as quickly as you can of course. Then bring it back to a simmer. A simmer (surface rippling) is all you need, not a wild rolling boil, splattering wort all over the place.
I'd only add an amount of extract in the beginning of the boil that's proportional to your boil volume. So if you only boil 1.5 gallons, add only 1/4 to a 1/3 of the extract. Use the liquid extract first. Bring back to a boil and add your 60' hops.
Boil for 60' and keep topping up (use boiling water if you can) as you're boiling off (evaporating) to keep roughly 1.5 gallons in your pot all the time.
When the 60' are up turn off the heat, then add and dissolve the rest of the extracts. Make sure to keep stirring well as you add them so they all dissolve and don't lie on the bottom. After adding the remainder of the extracts check the temp. If it's 150F or above put a lid on it and let sit for 10-15 minutes to pasteurize. If it dropped below 150F, slowly heat it back up to 150F, under constant stirring so it doesn't stick and scorch to the bottom. Then put a id on it and let sit for 10-15 minutes at 150F.
Then chill and transfer to your fermenter containing 2-3 gallons of water, and top off to the 5 gallon mark.
Note on Chlorine in brewing water:
If the water you use for brewing is municipal water, it likely contains Chlorine or Chloramines. They should be removed before brewing with it by treating it with Campden. Stir 1/4 crushed Campden tablet (or 1/16 tsp of K-Meta powder) into 5 gallons of water to dissolve it. It works almost immediately, done!