as far as modifying goes I would get an 89 or newer 5.0 because they came with the mass air computers and are easier to modify than the previous speed density, that is unless you want to build a race car then speed density is better

 

Avoid the 86's, the E6 heads are perhaps the WORST SBF head made for a 5.0. EVAR!!! If you try to do anything with stock heads, you're dead in the water with those lumps.

 

5.0's:
Cheaper and easier to mod. Better power off idle. Some people prefer the pushrod.

4.6's:
Love F/I. Alittle better on gas, alittle more reliable.

This topic could go on and on forever with opinions, facts, etc.....it's really up to personal preference and goals for the car.