first I am surprised you found a Marie there on the chopping block for discontinued planes.

if you want to read about the blue nosed P-51 here is 39 pages http://www.rcuniverse.com/forum/m_8623566/tm.htm

for the marie go to the warbirds section, there are several long posts about it.

the 2 planes are night and day apart, the marie is a 8.5lb plane with open bay wings,the retracts are light weight and although servicable, many like myself have chosen to replace them with upgrades and struts.

the blue nose is more like 11lbs, and has fully sheeted wings and a heavier set of upgraded retracts, foiled control surfaces similar to the Top Flight ARF/kit, I will let you do some reading for yourself.

I should preface this by saying the Marie is one of my favorite planes, it flys like a sport plane and floats in for landings, its a good starter warbird with retracts and is just a cool looking plane in general.

now for engines, mine is powered by a OS 91 surpass 4stroke, and has plenty of power to do just about any manuver I need it to do,there is no need to over power this plane it is very light, anything over a 120/125 4stroke and its waisted power, and really dangerous, since without some major strengthening it will rip its self apart,there are many posts about weak firewalls and the flat tail feathers failing when overpowering this plane.

I would recomend using a 4stroke since they hide well in the cowl, run inverted with little to no adjustment sound better in a warbird IMO,and in the case of OS have very little oil residue even after a full days flying.

if your going to run a 2 stroke they are a little more fussy when inverted and require the tank to be positioned just right for reliable running.

mounting it sideways isn't the worst thing for numerous reasons, using a pits style muffler, but your going to have a head sticking out of the cowl but not a big muffler hanging out the side something I can't stand on my planes.

the 75AX would be a good choice for a 2st, or a 91 maximum for the Marie, the 120AX is really over the top unless your working the model over for speed, because thats what your going to get, as well as needing to cut the cowl up quite a bit for cooling and clearance.

these are just my opinions of course here's mine