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I have a bit of experience with this stuff, just like Sam.
I ran a 1500 in my toledo, with a TH5 cam, 10:1 head and the larger valves, twin HS4s and teh 1500 twin cast manifold/downpipe it made 91bhp. However, I managed to destroy several cranks, less so after fitting an oil cooler and using millers CSS oil, but still one went and then I changed to a TR7 engine. Similar power, more torque, less stressed, more economical.
I also have experiebnce of 1300 engines.
They do rev better/easier, and even the big jornal ones last way better than a 1500. The small bearing crank is a gem.
I would go for a mk3 spitfire cam, 9.75 CR and definately use the Toledo log manifold with a single HS6/HIF44 carb.
A 4 branch exhaust manifiold is "difficult" and teh simple answer will be use the 1500 cast manifold/downpipe (but use 3/8 or M10 cap heads as they can be done up good and tight, much better than the weedy 5/16 studs)
The car will never be proper quick with the OHV engine, but could be much perkier with a few well chosen mods that won't cost a fortune or make the car difficult to drive.
(I had a mk3 spit with a 110+bhp 1300, twin dellortos, fast road 83 cam, roller rockers etc etc, but the buyers wanted std. I fitted a rebuilt std herald GE engine, a pair of std mk3 carbs etc, and the car was much nicer to drive 90% of the time)
I initially liked the idea of the 1500 cast manifold, but got put off by reading forum stories of it constantly blowing gaskets. However I saw a note about "sleeving" the joint to the downpipe, does this rectify the issue.
Would I also need the corresponding downpipe or could I get an adaptor made to the current exhaust (and would I need to increase the diameter of the rest of the exhaust system?).
You would need the downpipe too, unless somebody can make a copy. The twin pipes are quite long.
I fixed the blowing issue by filing the manifold and downpip faces flat (they were VERY lumpy) and using m10 capheads. Never an issue again.
However, I have remembered an issue. The twin cast manifold will NOT fit with a single carb inlet . I tried, and failed (apologies, this was over a decade ago) I even tried hacking bits off the inlet, and grinding the exhaust manifold.
For ease, do the engine etc, and keep the existing exhaust system? You will see the vast majority of the gains. MK3 spit is 75bhp, toledo 60? That is a big jump.