How to get 1500+HP

Reliable 1500 HP is easily done at low rpms with diesel's.
Gasoline engines with power adders + high rpms. Not as reliable but
likely more the kind of fun your question is directed.
I would have to go the nostalgic route with a 671+1st or 2nd gen. Chrysler Hemi.
It has proven itself capable over and over for decades.
I am from the over 50 crowd that still enjoys the old methods best.