If you fixed the headtube of your bike in a giant immobilizing vise, and pushed up on the rear wheel, a longer wheel base would generate proportionally greater leverage. However in the case of an extracycle, when the rear wheel pushes up, the rear axle goes up less high, which should be a gentler shock to the frame. Nothing in the original frame is fixed to allow this leverage to do anything nasty to the forward triangle.
What is quite different in an extracycle is that the load in the rear drops is not isolated there. The drops are a pivot point around which the rear extra frame pivots, except that struts restrain it. This could create weird loads ahead of the rear triangle, if these forward forces are not braced by a well placed strut. I can't see your full photo, but it appears to brace these loads to the bottom bracket, in which case they should not travel forward to the front "triangle".
As long as your weight is not excessive for normal use in the frame style chosen, and as long as you do not use a bike so converted to do harsh stuff the original frame couldn't have withstood, you should be OK.
However, I don't even play an engineer on TV.