Yes you can use whatever engine you want as a marine engine. It would be best to use a closed cooling system but not absolutely necessary, you will need an additional water pump that you could get from a mercruser or get a new jabsco item.

I would modify an l-series 5 speed good luck breaking that with an a-series

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I'm running a megasquirt to and love it, went from an ms1 with a dizzy to a ms3pro with 4 ls1 coils. should have done coil packs in the first place as the dizzy was the week link in my set up and took up space in an already over full engine bay.

You could take it to swmotorsport in Windsor or send bigE a p.m

Go for it I to had a d21 vg30 it was a mad car.

do you have the ego correction turned off? you need to have anything that can influence your tune turned off until you get a good state of tune other wise you will just chase your tale, also check the warm up enrichment isn't adding to much fuel.
if you are using itb's and a big cam you may not be getting a steady manifold signal so you may need to join the itb's with a synchronising hose and maybe put a restriction in the line. alfa/n setting is an option as well altho its a compromise.
is it fuel and spark or fuel only?

http://www.msextra.com/forums/index.p ... 2d4e84d01f65137a77087bdae

http://www.msextra.com/doc/index.html

good job on the gearbox x member mounts, me likey

They sound like the standard sedan belts. that's what's in mine and it gets rego in NSW. Do you have a pic?

i got plush pile from true fit carpet. it has a little room around the hump but the under lay will fill that up nicely