I used to be involved in hydros, and built or assisted in building everything from Crosleys to Merlins. I see that you aren't unafraid of building or rebuilding. One of the engines you mention is the 3.0 Merc. The original motor was the 153 Chevy ll. It was used a lot in the 2.5 liter hydro class as well as in Sprint cars. The architecture of the 153 is the same as the Chevy six minus two holes, and bore and stroke was 1/2 of a 307 v8. Connecting rods same as the 6, and Pistons the same as the 6 and v8. They were cheep to build and robust enough for hydro racing, the equal to a 3 1/2 min Dyno run, except when you bounced the prop free. Although we only dyno'd 230 hp, others claimed power near 300. The 3.0 is built to run wide open, all day long, at 4800 rpm. Say "Robust". With cam,induction,and headers it is easily capable of the power of most of the v6 you mention and there would be a fun part of using an old pushrod motor that dates back to 1962.
An engine you might consider is the 215 Buick and Olds, as well as the Rover variants up to 4.6 liter. Lighter than all the v6 engines you mention, and easily modified. Because of Rover building them for years, parts are not unobtainable. The 215s were used in the 225 hydro class. Running astronomical rpm, the 215s were by far the sweetest engines to hear out on the lakes. Another: a Chevy power book of the '90s was dedicated to racing with the 2.8/3.1/3.4 engines. A 60 degree motor which is way more compact than the 90degree v6s. you can stroke the 2.8 and 3.1 to 3.4, and production alloy heads were on some. Last, the 5.3ls motor with the alloy block is lighter than a 4.3,and much, much, more powerful.
Have fun!
Mike