Arif,
Firstly, if you can manage to get the old speedo and tach out then you're doing very well. Those wing nuts can be VERY tight after years and years. Once out then it's easy. But trust me the whole job would be easier with the dash out, esp. the three pod gauges.
As for Mech vs elec.: Mech are ususally more accurate, and the either work or don't, so you always know where you stand. On the other hand, they need bigger holes in the firewall to get the fittings through. The electric gauges are easier and neater (?) to install. I've never had any problems with electric gauges and I love the ease of installation.
I generally use a mechanical for 'oil pressure', just because that's the most important gauge that needs to work and work accurately. The rest I use electric.
You may find in car fuel pressure very expensive, that's why I'm not using one. But nice to have.
HTH.
Tim