Bravo is a very good drive and can easily handle 500HP but the real rating is in torque (most folks can't/don't do the conversion). If I recall correctly a Bravo 1 can handle about 550 ft-lbs and a Bravo 1X can handle about 650. Alpha starts falling apart 350-400. Bravo and Alpha use the same hole but the Bravo has 2 additional bolt holes. You will need a water pump with a Bravo since they do not have an internal one. Here is some info on the evolution of the drives:
https://www.go-fast.com/Mercury-Merc...uction-Changes
Make sure you use a marine alternator since the automotive ones are NOT ignition protected - ignition in the bilge= bad day.
Definitely go closed cooling and delete the VVT. Make sure you have a cam that gives low end torque (truck style) so you get better economy.
Like the Chevy small blocks, all the LS motors have the same block externally and just change bore/stroke. I see you have a free 5.3 but think about a 6.0 - maybe a L96 or LY6 if you can find it (even though those have iron blocks - it only adds about 75 lbs) or an aluminum 6.2.
Make sure you put on a remote oil filter. I would use aluminum exhaust manifolds with replaceable anodes. I have a source for those manifolds somewhere in my computer if you want me to run it down.