The Athans Spring Thing should work.  Rather than have one end of the spring attached to the motor mount as you would on a bladder fed model just attach it to the exhaust port with a bolt through the rear muffler lug.  The spring will have to be short enough to expand to allow the fuel tubing to pass through without hitting the body.  Drill a hole through the body and attach the fishing line to the other end of the spring and run it out to the leadouts as you would with a normal bladder fed model.  As with the bladder system, the flying lines keep the spring open and allow fuel flow during normal flight.  Since the line to the leadouts will be on the bottom of the wing you'll have to install a pin in the spar to hold the spring open for fuel flow during starting and takeoff.  There are a number of flyers using this system at Whittier, take a look at the F2d setup and modify to work on your profile model.  Why anybody would use a profile model with metal tank in Speed Limit Combat is beyond the scope of this reply. 

Lopez and Rudner will know what to do.  Ask them.  We fly 80 mph combat here (measured with streamer off or on).  Everybody uses shutoffs.  They work, even with a misguided proscription against certain designs. 

Why use suction feed?  

Thanks for the feedback so far and appreciate your response to this post. My original intention be known that I choose 75mph to avoid the shut-off rule but have found it not to be "Politically Correct" at our site.

I have witnessed flying in this class over the last couple of years at the Southwestern Regional Tucson AZ event ED'd by Jeff Hanaur with participants running profile designs running on suction so we would need to ask these folks why suction. They're having fun and participating just the same and are the kind of folks to "run what they brung". Since their field is an ideal location this event doesn't require shut-offs due to the speed limit as per AMA combat rules they don't use them. I am hoping to get those same guys and anyone else to come out and fly our events that at our field will be required to have shut-offs due to the close proximity of the adjacent heavily traveled freeway. I hope to engineer a solution to include these folks albeit very few but they're participating just the same.

Although I might be tasked with mission impossible the Athan's spring shut-off idea would seem to be the most logical place to start along with putting together something to reliably run on suction for R&D. All of this effort is to try to grow this event in So Cal dealing with our unique situation of sharing the field with some top notch F2D guys that fear a mishap could spell doom for our site.  It is those same guys that I will go to with this Athan's spring idea for this purpose to see what we can come-up with. For the last couple of years they have switched to magnetic and centrifugal shut-offs as I don't see the spring shut-offs in-use. I have one of Rich Lopez's old F2D combat planes from a couple years back with remnants of fishing line on the lead-outs that certainly seemed to be rigged at one time with a spring shut-off. Our local guys along with the folks posting here are brilliant and certainly we'll figure out a solution to work for these pilots to fly and grow our events.

Thanks again for your valued feedback and I'll share my results as they develop.  

I used a version of Roy Glen's shut off that used a short set of lines connected to the lead outs (or lines) to the pinch mechanism on slow combat stuff the year it was required in slow. It was one of the few that worked reliably that I remember.
LD