I’ve got a cockamamie idea to replace a single big lock with TRS drive with two Mercruiser V8s. Would love to hear suggestions...without dropping a fortune on exotic engines, what’s a doable package?

outboards weigh even less, then you'd be filling the transom hole (vs filling and cutting 2 new holes and stringers /etc) and bolting on 2 used 250HPs?

If I was looking at your conversion, I'd be looking at outboards... think of the new storage you'll have.

Small blocks with aluminum heads/intake/and aluminum afmkt exhaust aren't too expensive compared to other things in the boat word.

I'll leave it at that for now because you have given us little information on what boat and what you want to accomplish and what the budget is around.

You will probably have 10k just in glass work to convert it.

10k? 4 stringers, fill transom hole, couple gallons of resin....

ive got a 21’ Donzi hornet 3 as the starting point....

Given the hull you're starting with, better budget some money for KY Jelly or a large shoe horn to get a pair of V8's in there. You might want to look at a few other factory rigged, narrow beam boats with twins to see what a royal pain they are to rig and work on. The 28' Checkmate and 29' / 32' Fountains with twins bring back some bad memories for me. You're also probably going to need a large hump / scoop where your hatch currently sits because they're going to sit so much higher.

Nothing is impossible, just takes more money right? LOL
IMHO, I would think you would be money ahead if you started with a boat that had twins.

Too Stroked your post beat me to it! Lol

Take your budget and just buy a proper 502... at the end of the day it's still a 21 ' hornet, with twins it would be a HEAVY a$$ed 21' Hornet that would be impossible to work on.

502/ B1 is a proven platform in the 21'Donzi classic... but definitely need the HYD steering to run hard in all conditions safely (IMHO - have run with and w/o and the HYD is DEFINITELY a sound investment)

Staggered twin Mercruiser 4.3 V6 with Alpha drives might fit better and would be the lightest setup I can think of and wouldn't be crazy expensive.