You should be able to find a good 5.4 that doesn't need rebuilt for 400 or less. Got mine from wrecked truck for $100 And don't think you'll get enough for your 4.6 to buy and rebuild the 5.4. A stock rebuild is going to be 1500$ + These mod motors are not cheap to build u should just look into a 4v swap on your 4.6 I've seen head/intake combos for 200-500 everyday
What trans do u have? Since ur a 96 that would make u the first yr of the 4.6l so ull prolly need a pcm out of a 95 or older or u can go carb and skip alot of electrical bs and ull prolly need new engine mounts and a trans bell housing to bolt up to the 5.8 that is if u have a t5 trans if u dont have a t5 or t56 ull want to get one (the t56 6speed can be built to handle over a 1000 hp) and depending on how u build a 351w ur good for 300-1200hp so deciding what kind of hp u want is going to determan were and how to save money in ur build ex if u can use a t5 trans or if u built it to strong and need a t56. The body style of ur car is very similar to mine and a dart block 351w is whats going in my ride. U dont need to cut up ur engine bay or fire wall at all to fit this small block in just need new engine mounts.