Ok for a first generation RX-7 here is rx7v8.com reply:
Q. I have a 1st gen RX-7 and would like information on a V8 conversion, using a Chevy 350.
A. You will need a hood with a scoop, or a hood scoop.
We recommend changing the front coil springs to compensate for the added engine weight. We also install gas shock inserts at the same time, for all our conversions. We use a Chevy transmission, and we recommend changing the rear axle to a Chevy rear axle, 2.73 or 3.08 ratio. This will give you excellent gas mileage and good performance, without over-revving the engine at highway speeds.
The rotary rear axle would be the weakest link, and at 70 mph with a 3.90 or 4.10 Mazda rear axle, the engine would be revving at 4000 rpm, which is too high for sustained driving.
Here is a list of the parts we use for the conversion:
Motor Mount Kit, including Distributor cutaway cap for firewall $400
GM Rear Axle $1150
10' #1 battery cable $50
Disconnect switch (highly recommended for security) $60
Custom drive shaft $250
Custom 4-core Radiator $300
Custom Rad Saddles, top and bottom for V-8 rad $75
Rad hoses (molded) with T-section filler cap $132
Electric fan, 16", curved blade, 2300 cfm air flow $200
Fan Sensor $25
Fan Relay $35
225 lb 10" Coil springs $250
Gas inserts $130
Exhaust $600-800
Hood, fiberglass inside and out with factory hinges and latch, primed $650
OR Hood scoop, primed $175
Here is a brief outline of some of the labour required:
Pull motor, transmission and rear axle
Modify struts
Remove seats & carpet to bolt in tranny crossmember retaining plates
Install engine & tranny
Install transmission cooler
Install rad, fan & T fitting
Hook up electrical
Install rear axle
Move battery to back
Reprinted from:
http://rx7v8.com