I think LX stands for 4-barrel.
Yes, an L&S Stern Jack WILL add stress to the transom assembly, but not to any "borderline" level. The gimbal mount will be the weak link, but there again, not near a dangerous level (the same gimbal has been used on 330hp big blocks).
An Edelbrock 4-barrel Performer intake and a Holley 600 marine carb along with a stubstack and a tall flame arrestor...
Those additions may bring you to the 210-215 horse level.
Advancing timing a couple of degrees may help.
Aside from those items, you start spending money in big hunks. At that time, it would be cheaper to get a rebuilt 350ci.