I absolutely concur with everything said by Roger in post #93. The stock HEI from GM is hard to beat for up to 8,000 RPM and many of them are still in service after nearly 50 years. A genuine GM replacement would be fine (probably $450). If you opt for aftermarket, DUI is my first choice ($275- $300) followed by MSD ($250). Don't know what it will take to ship to Sri Lanka so if you can find in country, go with any of these three.

Any of these should be a direct drop-in replacement and should a tight fit to the manifold. There will be some movement in the shaft to accommodate advance. Set initial timing at 12 degrees (with all vacuum off and plugged at manifold) and then drive it like you stole it.

Best of Luck,
Glenn