I'm looking for tips. Going from 3" lap belt to Sparco 4pt belts.
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You'll need a substantial anchor point for the shoulder belts, ask Carey how they usually do that.
I asked him how he did it back when I dropped my car off back when I moved from NY to Utah. He said they mount a rod or bar across the firewall. That's probably what I'll do, but I was looking for alternatives.
My seats are taller than this, coming right up to the bottom of the trim strip.
The shoulder straps on these belts are, like, 8' long. I was thinking about just anchoring them to the floor behind the seat. Or running a bar between those two frame members and anchoring them to that
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Don't do that, the belts need to be mounted straight back from the top of your shoulders.
If you mount them to the floor, can you say spinal compression?
I ran the crotch belt through the seam in the seat bottom. My wife sewed it up.
I know crotch belts aren't comfortable, but they really keep you safe in an accident.
And why do they call them anti-submarine belts? It's not like you don't know the belt is in your CROTCH LOL!
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Understood. I was thinking of anchoring the eye bolt in the floor and running the straps up through footmans loops just below the trim strip, backed by a full-width extrusion on the other side of the firewall.
My thinking being drilling 4 small holes the through the firewall would be less traumatic than cutting 4x3" slots. Mostly because that would expose the nylon straps to the oily/heat goodness that is my engine compartment.
Both my seats are right up against the firewall. The eye-bolt and clip is at least 3". There's plenty of room for it on the floor. Behind my seat, not so much.
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According to GSU's HyperPhysics Project, a 160 lb person—wearing a seat belt and traveling at only 30 miles per hour—experiences around 30 g's of force in a front-end collision with a fixed object. That's 2.4 tons of force acting on the body!
If the belts are just for looks, go for it. If you want them to keep you in the car do what Carey does.
Agreed. Mounting shoulder belts to the floor and expecting a half-assed loop to carry the impact load is well, (put whatever word you want). I'll go with not smart.
The shoulder belts need to go straight back and be attached to something substantial, such as 1.5" or 1.75" DOM tubing welded to other frame members to take the load.
At 50-60 miles per hour, the g-force on you and the belts is quite high, on the order of 180 g. I'll bet those floor-mounted belts and firewall-mounted "footman loops" would rip right out.
The other part of that equation is WEARING them, every time you drive. Yeah, it takes a few extra seconds to attach 5 points, but they keep you safe.
I'm not convinced that 4 point belts will hold you, especially given the angle of the seat typical to Spyders.
Not really half-assed loops, as you call them, especially if they are anchored across the firewall to spread the load, but your post made me realize another factor I'd overlooked: Nylon straps lose 1/2 their load rating with every bend.
The fail point would probably be the footman's loops tearing the straps, not the loops failing.
My biggest concern, something I realized after switching my Speedster screen for the lexan one, is that it's somewhat of a guillotine. A proper lap belt will keep my head from hitting that.
Yeah, every harness bar I've ever seen is 1.5" round tube or bigger. That's got to decrease the stress on the belts.
I promised you rear suspension pics, but haven't taken any photos.
There are a bunch of decent ones here in this Seduction ad, same as Vintage trailing arms, because Vintage made them.
Thanks Danny. Back when I was working, I had a rudimentary understanding of all this stuff, because I used to hang tons (literally) of speakers above people's heads. Not so much now.
OTOH, there's this. When I started pondering, I got worried that, unlike most 3" lap belts, there wasn't a certification label on them anywhere. But they were in the installation instructions
As as for the suspension, it just as well. I realized it was a lot of work to reinvent the (rear) wheels. I'm going to go with SwayAway adjustable spring plates and a front sway bar. BTW, either you or Jimmy mentioned that a standard Beam sway bar won't work on a Spyder front end. Is that true?
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Never mind. 
I just googled a frame pic. You'd think with how much time I've spent laying in my back under the front end, I'd remember how it was put together.
I modded a VW Bug sway bar to fit between the trailing arms and attach on the lower. Danny made his from a Porsche bar and some steel (as I recall). There are several ways to skin this cat.