The only 4 bolt 305 in existence was in some Canadian heavy trucks. And they were modified to 4 bolts, they didn't come that way.
The eighth digit of the VIN will be H if it has a 305 in it.
If you can get a casting number that will tell us what motor it is. It is very unlikely (.0000000000001%) that it is a 4 bolt 305.
I've been researching this lately. I actually plan on rebuilding my old 305 and putting it in place of my 350. Then I'll rebuild my 350 and put it back in place of the 305. It's a good thing motor swaps only take 6 or 7 hours.
Anyway here's what I'd do and the order I'd do them. #5 has two parts because you can choose one or another.
1. Tune your carburetor. Get some DR rods for the secondaries. Play with the hangers, Somewhere between K and P should be good.
2. Edelbrock 2101 intake. Good for ten horses or so unless you've got a crappy equalized dual plane intake then it's worth more.
3. Headers. Good for just about ten horses. Find 1.5" primaries if you can because otherwise you'll sacrifice some low end torque.
4. 2" open spacer under the carburetor. This slowly generates horsepower but it doesn't break the 5hp mark until 3500 rpm. If you've got room, stick it in there. It's only $20. Worst case scenario is that you'll lose some bottom end torque and you'll have a $20 piece of phenolic plastic that would go great on that 400SBC powered Camaro you've been dreaming about.
5a. Find some 58cc heads if you don't already have them (I've never heard of them not being in a truck or blazer). Get them machined for 1.94 intake valves instead of stock 1.84. Don't forget a good set of undercut valves. This should bring your compression ratio up above 9.2:1. If you can, find a set that you can mill some material off so you can get the compression up a little higher yet without toying with new pistons.
5b. Camshaft. It seems that 108° works the best. I don't know anything more than that--I'm still researching.
6. Complete rebuild. Stick some higher compression pistons in it. Port the heads.
After both #5s and a rebuild you're supposed to be able to squeak [email protected]
,000rpm with a properly tuned carburetor.
Most 305s had between 7.5:1 and 9.2:1 compression ratios. You can get more ponies out of it by bumping up the compression to 10:1. You usually see a 2-3% horsepower increase for 1 point compression ratio increase.