This is an interesting topic here. I have a 250 with 7.4 MPI as well. I tried doing the same in hopes of maybe some performance gain- or? Did a lot of research and even bought a pair of EMI Thunders from a fellow member on MOC. Long story short they are not on my 7.4.
Ill sum up some experiences for you. The 7.4 is not really like the 454/502 motors. The terms kinda confuse folks due to displacement being 454ci in both the 7.4MPI and 454. That is about all they share. The 7.4MPI have what they call peanut port heads where the 454 has larger rectangle port heads. That is the first problem- headers you may find for a "454" will NOT fit the 7.4.
Now IF you were to strap a pair of aftermarket headers on the 7.4 you may run into another problem. Apparently the 7.4 is programed lean from factory for emissions. You throw on some more efficient flowing headers and you may lean out the motor past a safe limit. Do I know this for a fact? Never done it myself but I have read into it a lot. Feel free to hop on offshoreonly.com or powerboatnation.com or iboats.com. There is a ton of info. Some good some bad and some very questionable.
If you were looking to just replace the stock set up due to rust or whatever I would prob go with Mercruisers OR GLM. GLM is suppose to get you a little performance and they also offer an aluminum set that shaves of a decent amount of weight. I read in a performance boat article they actually did do a pretty good improvement on a boat running near sea level. GLM info
HERE.
Another thing to consider, Mariahs were built to be sporty, luxurious and cruise well. That is different than the go fast boats built for speed and performance. Just something to keep in mind.
Aftermarket Headers 7.4 MPI
