Caferacer says it all. I actually ride a mix of SPD-R and 7400's. I have a couple spare pairs of SPD-R's so for massed-start events I can ride the same pedals without straps and with slightly lighter tension, while for match sprint (my main focus) I can ride with higher tension and straps. I rode 7400's for years, but have to say that the SPD-R's actually hold better -- your foot doesn't try to lift out of the pedal the way they will on 7400's (which means you depend with 7400's on tight straps, while with SPD-R's you just need enough strap tension so that if you rotate your foot it won't go anywhere and will immediately reengage).
I TIG'd a strap on a couple pairs of pedals similar to what Tomity came up with. My only issue with them is that with the toe clip method, a modern laminated toe straps has enough stiffness to sit upright while it tends to flop around with Tomity's method. I tried shaping the welded-on bracket to have a narrow portion that holds the strap in place better, which helps, but isn't perfect. It's certainly an improvement over the toe clip method, which was an improvement over a piece of plumber's strap like we used to use.
If your feet are wide, you may really prefer the 7400's since the SPD-R's are rather narrow and you'll feel the straps on the sides of your feet. Plus, the big problem with SPD-R's is that you can't find many shoes with the cleat drilling any longer -- that's actually the biggest thing that keeps me going with 7400's (since they work with Look pattern drillings).
By the way, the prices on PD-7701 SPD-R pedals have more than doubled in the past year. For track, you do just fine with PD-7700's, which the road crowd detest but give superb retention. You can get them in very clean to new condition for $35-60, while 7701's are going for $125 and up. It's a much better deal than 7400's -- a NOS pair just sold (without cleats) on eBay for $255, and Business Cycles is now selling them for $289. I stocked up two years ago when they were $44 a pair. I guess there are enough track riders out there to create that much demand. Perhaps someone could come up with a decent track-dedicated pedal.
Not many people mention it, but the MKS Exa pedal is a solution to look at. It has a lock-in cleat (actuated by a lever on the pedal) but the cleat mounts on Look-pattern drillings. It's nicely done. You still want to use it with straps, but it's worth a look.
I've been unimpressed by the Speedplay track pedals. The tension isn't really higher -- it's just that the clip-out points on the rotation have a steep transition. If you feel it and stop rotating, that's fine. But it makes for very sudden clip-outs and then it's all over. Plus, enough riders (myself included) who pronate or otherwise rock inwards on their pedals end up scoring the axle with the edge of the cleat. I'm not sure I like the idea of grooving such a highly stressed piece of metal on the bike.