I didn't have any problems at all with the lower bolts or the oil sending unit, but I did need to file out the top holes for the plenum bolts a little just to make sure I didn't cross-thread any of those bolts by starting them at the wrong angle. I gained a nice amount of power from my home-ported SLP runners, although I also added a 58mm TB (worth maybe a few hp) and a better muffler (maybe 5 hp). You can see a graph comparing before/after runners/TB/muffler mods at:
http://www.erols.com/gcwestph/Dynocmp4.jpg
and my IROC webpage with all the mods and other graphs at:
http://users.erols.com/gcwestph/IROC.html
The small loss in low-end power/torque is more of a result of a little less timing and more fuel than due to the SLP runners. With 11.1 compression, I was fighting a little detonation in that range, and since the engine doesn't go below 4000 rpm after I shift it at the track anyways, it doesn't matter.